Added simple mbed SDK test linking against popular libraries

# Description
This change should enable mbed SDK's default Travis script to perform linking against popular libraries / tests so we can 

## Changes
1. Added new function and linking scheme data structure.
2. make.py was modified in separate commit to master to allow linking to given library and to enumerate tests' names with comma when using -n switch.
3. Added linking options for LPC1768 and most popular libraries.
4. Script should dynamically generate below commands:
```
make.py -m LPC1768 -t GCC_ARM -n MBED_2,MBED_10,MBED_11,MBED_15,MBED_16,MBED_17
make.py -m LPC1768 -t GCC_ARM --eth -n NET_1,NET_2,NET_3,NET_4
make.py -m LPC1768 -t GCC_ARM --fat -n MBED_A12,MBED_19,PERF_1,PERF_2,PERF_3
make.py -m LPC1768 -t GCC_ARM --rtos -n RTOS_1,RTOS_2,RTOS_3
make.py -m LPC1768 -t GCC_ARM --usb -n USB_1,USB_2,USB_3
```

## Testing
Tested with latest GCC_ARM 4.8:
(This is how it should look like in Travis' console)
```
C:\Work\b\r\r\build> workspace_tools\linking_travis.py
Executing: python workspace_tools/make.py -m LPC1768 -t GCC_ARM --eth -n NET_1,NET_2,NET_3,NET_4
Building project TCPCLIENT (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\NET_1\tcpclient.bin
Building project UDPCLIENT (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\NET_2\udpclient.bin
Building project TCP_SERVER (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\NET_3\tcp_server.bin
Building project TCP_CLIENT (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\NET_4\tcp_client.bin
Executing: python workspace_tools/make.py -m LPC1768 -t GCC_ARM --rtos -n RTOS_1,RTOS_2,RTOS_3
Building project BASIC (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\RTOS_1\basic.bin
Building project MUTEX (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\RTOS_2\mutex.bin
Building project SEMAPHORE (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\RTOS_3\semaphore.bin
Executing: python workspace_tools/make.py -m LPC1768 -t GCC_ARM --fat -n MBED_A12,MBED_19,PERF_1,PERF_2,PERF_3
Building project SD (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_A12\sd.bin
Building project DIR_SD (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_19\dir_sd.bin
Building project SD_PERF_STDIO (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\PERF_1\sd_perf_stdio.bin
Building project SD_PERF_FHANDLE (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\PERF_2\sd_perf_fhandle.bin
Building project SD_PERF_FATFS (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\PERF_3\sd_perf_fatfs.bin
Executing: python workspace_tools/make.py -m LPC1768 -t GCC_ARM --testlib -n MBED_2,MBED_10,MBED_11,MBED_15,MBED_16,MBED_17
Building project STDIO (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_2\stdio.bin
Building project HELLO (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_10\hello.bin
Building project TICKER (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_11\ticker.bin
Building project RPC (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_15\rpc.bin
Building project RTC (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_16\rtc.bin
Building project SERIAL_INTERRUPT_2 (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\MBED_17\serial_interrupt_2.bin
Executing: python workspace_tools/make.py -m LPC1768 -t GCC_ARM --usb -n USB_1,USB_2,USB_3
Building project BASIC (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\USB_1\basic.bin
Building project KEYBOARD (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\USB_2\keyboard.bin
Building project KEYBOARD (LPC1768, GCC_ARM)
Image: c:\Work\buildbot\regression\regression\build\build\test\LPC1768\GCC_ARM\USB_3\keyboard.bin
```