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
Revert "Fixed writable for LPC1768"

This reverts commit 19ae097c62efb66eeaa7c8ba8778a9b26d092fe3.
Fixed LPC1768 writable

Now if no flow control is used it will check if holding register is
empty, instead of complete UART is empty: The TxIrq uses the same
requirement.
Currently BufferedSerial (and everyone else trying something similar)
goes haywire because it doesn't just check if writable, but also if done
writing.

If flow control is enabled nothing is changed.
Merge pull request #194 from Sissors/master

LPC1768 writeable fix + KL25Z/KL46Z RTC pin function