core: align stack on a 32bit boundary

Fixes #1267.
SQUASH: fixes to stack-align fix
cpu: removed NAKED attribute from ISRs

- removed the __attribute__((naked)) from ISRs
- removed ISR_ENTER() and ISR_EXIT() macros
boards: fixeded UART defines for NRF borads

fixed for pca10000, pca10005, yunjia-51822
cpu/nrf51822: added interrupt mode to UART driver
tests/periph_uart_int: fixed formatting
boards: added missing STD_RX_BUFSIZE defines

added for:
- pca10000
- pca10005
- yunjia-nrf51822
cpu/nrf51822: made syscalls aware of uart0