Merged in ccxcz/nuttx/olimex-stm32-h407-v2 (pull request #518)

Olimex stm32-h407 serial support for the on-board UEXT connector (fixed style & defconfig)

* olimex-stm32-h407: Add USART6 for UEXT connector.

* olimex-stm32-h407: nsh_uext configuration and README update.

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