Fixed: mbed-os as an IAR library

Long build times got you down? Maybe you’ve tried building mbedos as a library in the past with weird bugs.

The issue is that, by default, ARMCC and IAR do NOT honor _weak vs non weak symbols when compiled into a library/archive. from ARM

IAR has a workaround command line switch for this, --whole_archive. Include your library with the “Extra Options” tab of the Linker options (instead of the Libraries tab).
Example: --whole archive mbed-os.a