optee-os: unbreak parsing when arch is not armv7a nor aarch64

When meta-arm is in the layer stack, parsing will break for unsupported archs,
like armv5, armv6, etc. Unbreak parsing by defining null default OPTEE_ARCH.

Signed-off-by: Denys Dmytriyenko <denys@ti.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-security/optee/optee-os_git.bb

@@ -23,6 +23,7 @@ S = "${WORKDIR}/git"
 OPTEEMACHINE ?= "${MACHINE}"
 OPTEEOUTPUTMACHINE ?= "${MACHINE}"
 
+OPTEE_ARCH = "null"
 OPTEE_ARCH_armv7a = "arm32"
 OPTEE_ARCH_aarch64 = "arm64"
 OPTEE_CORE = "${@d.getVar('OPTEE_ARCH').upper()}"