cpu: adjusted IRQ stack sizes for Cortex CPUs
Merge pull request #1954 from haukepetersen/fix_cortex_irqstacksize

cpu: adjusted IRQ stack sizes for Cortex CPUs
core: remove needless test in msg.c

`thread.c` initializes a thread with an empty message queue. `cib_put()`
will `return -1` for an empty CIB, so there is no need to test if
`thread->msg_array != NULL`.
initial support for c++ for this cpu/board
Merge pull request #2013 from Kijewski/msg-needless-test

core: remove needless test in msg.c
Merge pull request #1985 from BytesGalore/EasyCPPMixing_stm32f4discovery_support_fix

squashed, changed comment and go.