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.
tests: remove do-while(0) blocks for netdev tests or comment on them
Merge pull request #2154 from authmillenon/tests/cq/remove-do-while-0

tests: remove do-while(0) blocks for netdev tests or comment on them
x86: Fix `make term` for BOARD=qemu-i386

Fixes #2184.

QEMU has changed its command line arguments.
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
Merge pull request #2185 from Kijewski/issue-2184

x86: QEMU has changed its command line arguments
make: change escaped quotation marks

The '""' pattern is used so that the variable can be processed as a string
after the preprocessors evaluation. Change to \"\" instead, so that single
quotes can be used externally. The extra quotation marks are unneeded.

Related to #2213
Merge pull request #2227 from LudwigOrtmann/make-escape

make: change escape sequence