meta-facebook: bletchley: enable u-boot SPL

The latest u-boot no longer boots on AST2600 without enabling
the SPL.  Port the changes from commit
d84b7f4455406807637b7fecf9e96b73e17fcf62 and tested in QEMU.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I5ce3f3a97d44ae58c54e3d44251726a84fcc6098
diff --git a/meta-facebook/meta-bletchley/conf/machine/bletchley.conf b/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
index 2c7778d..8de7a54 100644
--- a/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
+++ b/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
@@ -2,8 +2,9 @@
 #KERNEL_DEVICETREE = "${KMACHINE}-bmc-facebook-${MACHINE}.dtb"
 KERNEL_DEVICETREE = "aspeed-ast2600-evb.dtb"
 
-UBOOT_MACHINE = "ast2600_openbmc_defconfig"
+UBOOT_MACHINE = "ast2600_openbmc_spl_defconfig"
 UBOOT_DEVICETREE = "ast2600a1-evb"
+SPL_BINARY = "spl/u-boot-spl.bin"
 
 require conf/machine/include/facebook-nohost.inc
 require conf/machine/include/ast2600.inc