nrf51: halt works better
CI: removed AC in extern C check
CI: replaced head with cat and filtered out tools for extern "C" check.
Add TERMFLAGS to info-build of Makefile.buildtests

As requested in #1559
core/msg: use disable/restoreIRQ

* needed to change internal `msg_send` to allow external disabling of interrupts
core/msg: coding convention cleanup
spark-core fix UART speed
Merge pull request #2073 from aep/master

nrf51: halt works better
Merge pull request #2086 from mehlis/spark-core-fix-uart-freq

spark-core fix UART speed
board/spark-core: enabled C++
Merge pull request #2032 from haukepetersen/fix_sparkcore_cpp

board/spark-core: enabled C++
Merge pull request #1887 from LudwigOrtmann/msg_new_irq_api

core/msg: use disable/restoreIRQ
Merge pull request #2080 from dangnhat/removed_AC_in_externc_check

CI: Extended extern C check
Merge pull request #2084 from x3ro/document-termflags

Add TERMFLAGS to info-build of Makefile.buildtests