Fixed some idle thread related stuff.

- Changed old MU_ prefix on some macro names to AR_.
- Removed MU_PRINT_SYSTEM_LOAD macro and related code.
- Made Kernel::s_systemLoad variable volatile. Without this, the IAR
  optimizer removes the entire load calculation loop core.
- Replaced code in idle thread to use the AT91C PMC module to put the
  chip to sleep with a WFI instruction intrinsic.
- Removed unused idle thread config macros from ar_named_object.cpp.
Added initial K60 support.

- Renamed original KL25 argon_test project files to argon_test_KL25.*.
- Created argon_test_K60 project. mbed sources are excluded from the
  build.
- Added 512KB_Pflash.icf linker config file.
- Removed bootloader blocks from 128KB_Pflash.icf linker config.
- Minor scuart.c update.
- Added K60 CMSIS files.
- Copied KL25 mbed HAL files for K60. Completely not working, yet.
- Removed LED code from basic_test.cpp. This will be added back in a
  new file in a different commit.
- Changes to debug_uart.cpp to get it to compile for the K60. Doesn’t
  actually output anything on the UART, yet.
Added accelerometer LED demo code.

- Split demo code out from basic_test.cpp into a new demo app. It
  doesn’t currently have a project file.
- Added mbed driver classes for MMA8451Q and TSI.