Update to mbed SDK revision 92
Major update to makefiles

These updates were made to mimic the TARGET_ and TOOLCHAIN_ directory
filtering that the Python based mbed SDK build system use.  Now when
building for a particular device you specify the target directories
that should be included in the build by listing them in the
TARGETS_FOR_DEVICE variable.  The makefiles will now exclude any
directories containing TARGET_ which don't match the provided list.
The makefiles also exclude non GCC and GCC_ARM toolchain directories.

Changes that impact per device makefiles (ie. LPC176x-device.mk):
* The *_TARGET_SRC (ie. HAL_TARGET_SRC, ...) variables are no longer
  required. The build file will automatically find source files in
  these directories (since they match the specified TARGETs).
* TARGETS_FOR_DEVICE is new and needs to be set to the list of
  TARGET_ directories that should be included in the build for this
  device (ie. TARGET_LPC1768, ...).
* GCC_DEFINES no longer contains the list of -DTARGET_ definitions but
  instead just builds them from the TARGETS_FOR_DEVICE list by
  prepending -D to each item.
Update README to mbed SDK revision 92