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
core: sched.c did not compile with DEBUG=1
boards/cc2538dk: allow the application's makefile to override the UPDATE_CCA parameter, for example:
CFLAGS += -DUPDATE_CCA=0
embunit: fix macros
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.
avr: define ssize_t
Remove special case for atmega in kernel_types.h
board/arduino-due: removed newlines in periph_conf
board/udoo: added RNG configuration
Merge pull request #1986 from Kijewski/smaller-sys_makefile

make: easify {sys,drivers}/Makefile
Merge pull request #1992 from haukepetersen/add_udoo_random

board/udoo: added RNG configuration
Merge pull request #1990 from Kijewski/avr-ssize_t

avr: define ssize_t
cpu/sam3x8e: added interrupt mode to uart driver
board/arduino-due: added missing UART defines

- added port defines to periph_conf.h
- added feature to Makefile.features
Merge pull request #1991 from haukepetersen/fix_artuinodue_periphconf

board/arduino-due: removed newlines in periph_conf
board/udoo: added missing UART defines

- added defines to periph_conf.h
- added UART feature to Makefile.features
cpu/sam3x8e: made UART0 work by editing syscalls
Merge pull request #1971 from Kijewski/assert-macros

embunit: fix macros
Merge pull request #1482 from haukepetersen/fix_sam3x_finished_uart

cpu/sam3x: completed low-level UART driver