riscv: binman: Add help message for missing blobs

Add the 'missing-msg' for more detailed output
on missing system firmware.

Signed-off-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Yu Chien Peter Lin <peterlin@andestech.com>
diff --git a/tools/binman/missing-blob-help b/tools/binman/missing-blob-help
index c61ca02..4448ac9 100644
--- a/tools/binman/missing-blob-help
+++ b/tools/binman/missing-blob-help
@@ -37,3 +37,9 @@
 tee-os:
 See the documentation for your board. You may need to build Open Portable
 Trusted Execution Environment (OP-TEE) with TEE=/path/to/tee.bin
+
+opensbi:
+See the documentation for your board. The OpenSBI git repo is at
+https://github.com/riscv/opensbi.git
+You may need to build fw_dynamic.bin first and re-build u-boot with
+OPENSBI=/path/to/fw_dynamic.bin