x86: Fix `make term` for BOARD=qemu-i386

Fixes #2184.

QEMU has changed its command line arguments.
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_()
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
Correct and robust management of returning ACKs for TXed packets