[serial] Explicitly initialise serial console UART to NULL

[serial] Explicitly initialise serial console UART to NULL

When debugging is enabled for the device tree or memory map parsing
code, the active serial console UART variable will be accessed during
early initialisation, before the .bss section has been zeroed.

Place this variable in the .data section (by providing an explicit
initialiser), so that reading this variable is well defined even
during early initialisation.

Signed-off-by: Michael Brown <mcb30@ipxe.org>