boards: adjusted TIMER_0 defines for NRF based b.

adjusted for
- pca10000
- pca10005
- yunjia-nrf51822
cpu/nrf51822: fixed timers

- changed TIMER_0 to be a 24-bit timer -> see Nordic PAN #32
- cleaned up timer initialization code
- added check if an interrupt channel was active
cpu: adjusted IRQ stack sizes for Cortex CPUs
cpu/stm32f0: fixed low-level timer driver
core: sched.c did not compile with DEBUG=1
c++: sys: add extern C to header files
Merge pull request #1793 from BytesGalore/extern_C_in_headers_sys

sys: add extern "C" to headers
nativenet: add missing include
core: always set sender_pid in msg_send_int
Merge pull request #1977 from authmillenon/nativenet-include

nativenet: add missing include
Merge pull request #1956 from haukepetersen/fix_stm32f0_timer

cpu/stm32f0: fixed low-level timer driver
6lowpan: free fragment buffer in error case
6lowpan: fix buffer overrun in decompression
IPv6: allow complete in ipv6_addr_init_prefix

If the specified prefix is a full IPv6 address, no bits remain.
Fixes a possible buffer overrun.
net_if: fix buffer overruns
6lowpan: do not overrun in context_remove
licenses: fix miss-spelled & missing boiler plates
Merge pull request #1974 from OlegHahm/cleanup_licenses

licenses: fix miss-spelled & missing boiler plates
boards/cc2538dk: allow the application's makefile to override the UPDATE_CCA parameter, for example:
CFLAGS += -DUPDATE_CCA=0
Allow the application's makefile to specify a custom linker script, for example:
LINKERSCRIPT = custom-linkerscript.ld