meta-facebook: bletchley: Disable socsec signing

Bletchley doesn't make use of the AST2600's hardware root-of-trust:

https://discord.com/channels/775381525260664832/867820390406422538/936755075920068759

Change-Id: I477d72cf1413fc0141714d863b22ac15637e259b
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/meta-facebook/meta-bletchley/conf/machine/bletchley.conf b/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
index c7407e8..174d3c5 100644
--- a/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
+++ b/meta-facebook/meta-bletchley/conf/machine/bletchley.conf
@@ -4,6 +4,7 @@
 UBOOT_MACHINE = "ast2600_openbmc_spl_defconfig"
 UBOOT_DEVICETREE = "ast2600a1-evb"
 SPL_BINARY = "spl/u-boot-spl.bin"
+SOCSEC_SIGN_ENABLE = "0"
 
 require conf/machine/include/facebook-nohost.inc
 require conf/machine/include/ast2600.inc