board/mbed_lpc1768: added UART configuration
cpu/lpc1768: added basic UART driver
vtimer: set custom msg_t.type with vtimer_set_msg

Currently, when using vtimer_set_msg the corresponding msg_t is filled
with the MSG_TIMER ("12345") type.
This approach makes it difficult to differentiate between incoming
messages via vtimer_set_msg.

In this PR I introduce another parameter for the vtimer_set_msg
function to specify a custom msg_t type.
board/mbed_lpc1768: changed binary ouput to .bin
board/mbed_lpc1768: added flash script