meta-google: Ignore configuring meta-tpm layer

Follow advice described in meta-tpm README to avoid warnings on each
bitbake invocation saying that the layer is not configured.

Change-Id: Ic7578ce4fe558bcae5204bb42b30f2247eff1c54
Signed-off-by: Vlad Sytchenko <vsytch@google.com>
diff --git a/meta-google/conf/distro/gbmc.conf b/meta-google/conf/distro/gbmc.conf
index 74c3d0e..aca4415 100644
--- a/meta-google/conf/distro/gbmc.conf
+++ b/meta-google/conf/distro/gbmc.conf
@@ -39,3 +39,6 @@
 
 # Remove dependencies on python3-core when using nftables
 PACKAGECONFIG:remove:pn-nftables = "python"
+
+# Do not configure TPM layer
+SKIP_META_TPM_SANITY_CHECK = "1"