cactus: select different stdout device at runtime

cactus is used as both primary and secondary VM and for debug logging
primary VM can access to UART while secodary VM's use hypervisor call to
SPM.

Based on VM id it will be decided whether to use UART or hypervisor call
for debug logging.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I97786b893c0156815969692582b4de62c1b568fd