Merged in spiriou/nuttx/stm32_lowputc (pull request #613)

arch/arm/src/stm32/lowputc: fix uart glitch when lowputc is enabled

Calling up_lowputc() when a character is in the shift register results
in corrupted character on stm32f1xx and stm32f205 cores.
TC status bit ensures that up_lowputc() waits for an on-going
transmission to complete before writing in TDR.

Approved-by: Gregory Nutt <gnutt@nuttx.org>