util/vboot_lib: Add description.md

Fixes lint-stable-025 error.

Change-Id: I4aa2b2a2ffca69f894a23d7487926016830c9e4f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50114
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/vboot_lib/description.md

@@ -0,0 +1 @@
+Support library for vboot in cbfstool.