Add support to specify payload UART

UART used by payload(such as tftf, Linux) may not be the same as the
primary UART. Set a run environment variable to track the payload
UART which is tracked to check if the test has finished sucessfully.

If not specified, payload UART is same as primary UART.

Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
Change-Id: I951eb0e6a09e31a92587a837b168c5084c508901
2 files changed