cpu: native: Add [v]fprintf to syscalls

New 'csma_sender' helper module

External packages that may use fprintf(stderr, ...) for logging will
link directly to the libc version of it, and for some reason that
results in the application crashing.

core: change msg.content.ptr type to void *

sixlowpan_print: fix dump for elided source/destination address

gnrc_sixlowpan_iphc: propagate UDP decode error

gnrc_sixlowpan_iphc: fix compile error for debug output

drivers: netdev2: remove netdev2 event_callback isr_arg

cpu: add capability to show stack usage of ISR

sys: fmt: add fmt_u64_dec(), print_u64_dec()

sys/shell: fix build issue when deactivating shell echo (iotlab)

pkg: split patch and download rule

git-cache: also fetch tags

cpu: Cast to unsigned int for argument bounds checks

cpu/cortexm_common: Remove register keyword from variables

make debug: start gdb quietly