Added option to build rpc library. closes #1426

[justbuchanan]

Here's my attempt at fixing #1426, which I filed earlier. This PR adds a --rpc option to build.py and make.py that does the same thing as similar flags for the other libraries.

Here's what I get when I run the build script now:

$ python2 workspace_tools/build.py --mcu=LPC1768 --tool="GCC_ARM" --rpc

[WARNING] Using default settings. Define your settings in the file "workspace_tools/private_settings.py" or in "./mbed_settings.py"
Building library CMSIS (LPC1768, GCC_ARM)
Building library MBED (LPC1768, GCC_ARM)
Building library RPC (LPC1768, GCC_ARM)

Completed in: (0.02)s

Build successes:
  * GCC_ARM::LPC1768

And the outputs in the build dir seem to show that it works:

$ tree build/rpc 
build/rpc
├── Arguments.h
├── mbed_rpc.h
├── parse_pins.h
├── RpcClasses.h
├── RPCFunction.h
├── rpc.h
├── RPCVariable.h
└── TARGET_LPC1768
    └── TOOLCHAIN_GCC_ARM
        └── librpc.a

2 directories, 8 files

[0xc0170]

Hi, can you please sign http://developer.mbed.org/contributor_agreement/?

[justbuchanan]

Sure thing, just did!

[0xc0170]

Thanks

LGTM