cmd: pxe: add board specific PXE default path

The list of PXE default paths contains ARCH and SOC specific paths, but
one PXE server can serve different board with the same ARCH and SOC.
This is the case for Turris Omnia and Turris Mox, where ARCH=arm and
SOC=mvebu.

If CONFIG_SYS_BOARD is defined, also try "default-$ARCH-$SOC-$BOARD"
path.

Signed-off-by: Marek Behún <marek.behun@nic.cz>