rainier and tacoma: Build U-Boot SPL

Build the u-boot and the SPL binary. The image_types_phosphor.bbclass
keys off these configs to concatenate them together into a single
file which is used during code update to update the device where
the SPL+U-Boot reside.

Change-Id: I1817f241da72a235a9e91514ee158236340f0d40
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/conf/machine/witherspoon-tacoma.conf b/conf/machine/witherspoon-tacoma.conf
index 8f2d362..bd8d711 100644
--- a/conf/machine/witherspoon-tacoma.conf
+++ b/conf/machine/witherspoon-tacoma.conf
@@ -1,6 +1,7 @@
 KERNEL_DEVICETREE = "aspeed-bmc-opp-tacoma.dtb"
-UBOOT_MACHINE = "ast2600_openbmc_defconfig"
+UBOOT_MACHINE = "ast2600_openbmc_spl_defconfig"
 UBOOT_DEVICETREE = "ast2600-tacoma"
+SPL_BINARY = "spl/u-boot-spl.bin"
 
 OBMC_POWER_SUPPLY_INSTANCES = "0 1"