uboot-imx

test/py: Support setting up specific timeout

Large file transfers, flash erasing and more complicated tests
requires more time to finish. Provide a way to setup specific
timeout directly in test.

For example description for 50s test:
timeout = 50000
with u_boot_console.temporary_timeout(timeout):
  u_boot_console.run_command(...)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>