tests: fixed blacklisting for UART tests

changed from explicit blacklisting to feature based approach
boards: added missing UART feature

added for:
- airfy-beacon
- arduino-mega2560
- cc2538dk
- msbiot
- openmote
- pca1000[0|5]
- spark-core
- stm32f4discovery
- yunjia-nrf51822
Merge pull request #2165 from haukepetersen/fix_test_uart

test/periph_uart_xx: switched to feature based whitelisting