MIMIMAL: add uart

While uart could be detected completely through plug and play means, add
it here for two reasons. First, we don't do that from the loader, so
it's not available as a console. Second, even if we did do it from the
loader, there's a limitation in the system today that console drivers
must be compiled into the kernel because the console is selected before
external modules are linked into the kernel. Adding it only increases
the kernel size by ~14k as well.

Sponsored by:		Netflix
Idea liked by:		des, rpokala, brooks, jhb

sys/amd64/conf/MINIMAL

@@ -118,6 +118,10 @@ device		vt_vbefb
 
 device		agp			# support several AGP chipsets
 
+# Bring in 'uart' as well, since it can be a console driver and all console
+# drivers must be compiled into the kernel.
+device		uart
+
 # Pseudo devices.
 device		loop			# Network loopback
 device		padlock_rng		# VIA Padlock RNG