core: introduce `msg_sent_by_int()`

msg_send_int() sets `m->sender_pid = target_pid`. This was used to flag a
message as having been sent by an ISR.

This PR introduces a static inline function `msg_sent_by_int()` and a
specific define for this purpose.
Robust use of TimerA for 'hwtimer' module
Use TimerB for 'hwtimer' module implementation
Merge pull request #1791 from Kijewski/msg_send_int-typo

core: introduce `msg_sent_by_int()`
Merge pull request #1113 from rousselk/msp430-use-timerB

Use Timer B on MSP430 architecture
drivers/xbee: First version of XBee 802.15.4 modem driver