Fixed writable for LPC1768

Bit 6 (0x40) is true if the peripheral is completely empty.
Bit 5 (0x20) is true if the transmit hold register is empty, which is
sufficient for it to be writable.

The TxIrq uses the same requirement, so now it works together and
BufferedSerial doesn't go haywire on LPC1768
Fixed KL25Z/KL46Z RTC-IN pin function

It used to set it to alternate function 2, which is I2C, and still works
fine in all my tests. Not intended though, and might be giving others
issues. 1 is the RTC_CLKIN