sys: Simplify enabling EARLY_PRINTF uarts

Support selecting the early uart with "options EARLY_PRINTF=foo" in
the kernel configuration file. This allows us to not have to change
source files when enabling EARLY_PRINTF, simplifying enabling it.

New uart drivers can be enabled by defining a new early_printf_foo
value to be unique, then using "#if CHECK_EARLY_PRINTF(foo)" to decide
when to enable the uart.

While here add pl011 early printf support.

Reviewed by: imp (earlier version)
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43360