sama5d3_explained: add udc and g-serial to MACHINE_ESSENTIAL_RRECOMMENDS

This will ensure images will pick them up. The 'kernel-module-u-serial'
is added to solve a bug in the kernel.bbclass dependency code. The
g_serial module needs it, but doesn't depend on it.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
linux-yocto-custom: enable PRINTK_TIME for sama5d3_xplained

This will add a timestamp to each line of kernel output.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>