examples/er-rest-example: Add radio sensor.

Added back the RSSI measurement functionality from Contiki 2.7.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
ldscripts: Place .preinit_array, .init_array and .fini_array in ROM.

.preinit_array will not work unless it is placed in ROM since it is
called before the C initialization which copies the .data section to
RAM. .init_array and .fini_array contains pointers to static constructors
and destructors, at the moment I do not see a use case where they need to
be writable at runtime.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
cpu/arm/k60: Handle init array in startup.

.init_array and .preinit_array sections are expected to contain
pointers to functions to be called during C library initialization.

- .preinit_array entries are called before .data and .bss are
  initialized and must not rely on any static or global variables.

- .init_array is called after .data is copied to RAM and .bss is zeroed
  out.

- .fini_array is still a TODO, however not a very high priority.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>