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
core: sched.c did not compile with DEBUG=1
core: always set sender_pid in msg_send_int
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
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
embunit: fix macros
net: fix buffer overrun in socket initialization
Merge pull request #1982 from locicontrols/custom-linkerscript

Allow the application's makefile to specify a custom linker script
Merge pull request #1980 from OlegHahm/coverity_fixes

net: fixes some finding from coverity
Merge pull request #1978 from OlegHahm/msg_send_int_set_pid

core: always set sender_pid in msg_send_int
make: easify {sys,drivers}/Makefile

Currently you need to add every new sys and driver module into the
respective Makefile. This requires rebasing if another module was merged
in the meantime.

This PR allows you to omit the entry to {sys,drivers}/Makefile, if the
subfolder has the same name as the module name, which should be sensible
in most cases.
exchanged BOARD_BLACKLIST with FEATURES_REQUIRED in the example Makefile
added cpp as feature for board native
Merge pull request #1988 from BytesGalore/riot_and_cpp_blacklist_to_required_feature

examples/riot_and_cpp: exchanged BOARD_BLACKLIST with FEATURES_REQUIRED