Only compile in uart_cpu_$MACHINE.c if it exists. I'm not sure how

useful it will be, but we really need to be keying off something other
than MACHINE for this anyway since on arm and mips we have lots of
these running around (one for each SoC family)...

sys/modules/Makefile

@@ -280,7 +280,7 @@ SUBDIR=	${_3dfx} \
 	twe \
 	tx \
 	txp \
-	${_uart} \
+	uart \
 	ubsec \
 	udf \
 	udf_iconv \
@@ -323,8 +323,6 @@ _vpo=		vpo
 # no BUS_SPACE_UNSPECIFIED
 # No barrier instruction support (specific to this driver)
 _sym=		sym
-# no uart_cpu_$MACHINE_ARCH
-_uart=		uart
 # intr_disable() is a macro, causes problems
 _cxgb=		cxgb
 .endif

sys/modules/uart/Makefile

@@ -16,7 +16,7 @@ SRCS=	uart_bus_acpi.c ${uart_bus_ebus} uart_bus_isa.c uart_bus_pccard.c \
 	uart_if.c uart_if.h uart_subr.c uart_tty.c
 .if ${MACHINE} == "sun4v"
 SRCS+=	uart_cpu_sparc64.c
-.else
+.elif exists(${CURDIR}/uart_cpu_${MACHINE}.c)
 SRCS+=	uart_cpu_${MACHINE}.c
 .endif
 SRCS+=	bus_if.h card_if.h device_if.h isa_if.h ${ofw_bus_if} pci_if.h \