[EXPORT] commandline options for export_test.py similar to build.py

This is a suggestion to use the tool export_test.py in a similar way like e.g. build.py. The reason to change the behaviour was the development of an gcc_arm toolchain/exporter for Nucleo_F334. The hal of this board currently does not allow to link with the rtos library. Both define a SysTick_Handler. 
With command line options, similar to build.py, the export can be limited now to specific toolchains, targets and/or export projects.
Without a command line option the behaviour is the same as before.

Here are the options:

Usage: export_test.py [options]

Options:
-h, --help            show this help message and exit
-a, --all             export all projects and MCU to all toolchains
-m MCU, --mcu=MCU
     export for the given MCU (ARCH_PRO, DISCO_F051R8, ... STM32F407)
-t TOOLCHAIN, --tool=TOOLCHAIN
     export to the given TOOLCHAIN (codesourcery, coide, ... uvision)
-p PROJECT, --project=PROJECT
     export the given PROJECT (MBED_BLINKY, RTOS_MBED_BASIC)