[build] Add predefined shortcut for Raspberry Pi builds

Add a build shortcut "rpi", allowing for e.g.

  make CONFIG=rpi CROSS=aarch64-linux-gnu- bin-arm64-efi/rpi.efi

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/Makefile

 DRIVERS_ipxe	= $(DRIVERS_net) $(DRIVERS_infiniband) \
 		  $(DRIVERS_xen) $(DRIVERS_hyperv)
 
+# Raspberry Pi
+#
+DRIVERS_rpi	= smsc95xx lan78xx
+
 ###############################################################################
 #
 # Drag in the bulk of the build system