uboot-mediatek: import patches improving menu navigation

uboot-mediatek: import patches improving menu navigation

Using the arrow keys to navigate the U-Boot menu often leads to being
dropped into the U-Boot shell unexpectedly.
This can be prevented in most cases by improving the logic to detect the
arrow key ESC sequence and only reprinting the menu if actually needed.
Also enable CONFIG_SERIAL_RX_BUFFER for all boards as it helps preventing
the remaining cases.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>