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
Use custom assembly code instead of MSP GCC primitives for dINT()/eINT()
and all other functions that enable or disable interrupts
Ensure interrupts are enabled before going in low-power mode
Ensure only GIE bit is touched in splx_()
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
cpu/lpc1768: fixed UART PCLK selection
board/mbed_lpc1768: added CLKSEL to periph_conf
Merge pull request #2222 from haukepetersen/fix_lpc1768_uartclk

cpu/lpc1768: fixed UART PCLK selection
stm32f1: make function definition match declaration
Merge pull request #2223 from kaspar030/stm32f1_fix_function_definition

stm32f1: make function definition match declaration
Merge pull request #2214 from rousselk/msp430-fix-intr

Msp430 misc interrupt-related fixes