initfs: add missing dependencies

Add a missing runtime dependency on base-utils.

Found the missing dependency on the shell while comparing initfs to
OE-Core's initrdscripts/initramfs-framework.

Change-Id: Ia43495245f37e7f48c5508ee42540abea7ab0659
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>