New local build setup.

See https://community.spark.io/t/tweak-request-to-core-firmware-src-build-mk/2900/14

Put whatever subdirectories you like in the applications and libraries directory, they won't conflict with the upstream core-firmware git repo. The directory structure should be, for example:

```
core-firmware/libraries/
   bmp085/
      bmp085.cpp
      bmp085.h

core-firmware/applications/bmp085_test/
   bmp085_test.cpp
   build.mk
```

Then bmp085_test/build.mk should be, for example:

```
ifeq ($(APP),bmp085_test)
CPPSRC += applications/bmp085_test/bmp085_test.cpp     # application
CPPSRC += libraries/bmp085/bmp085.cpp                  # library
endif
```

As per the cloud IDE, you should include libraries with their directory name, for example in bmp085_test.cpp:

```
#include "bmp085/bmp085.h"
```

When you want to compile a specific application, call ```make``` and pass in the application's name, for example:

```
make APP=bmp085_test
```
Or to simply compile the default tinker application, omit the APP flag, for example:

```
make clean all
```

If you wish to change the default application, or don't want to pass in the APP flag each time, then edit the APP in applications/build.mk, for example:

```
APP=bmp085_test
```

Then just call make.