Rather than add runtime overhead of installing completion handlers, do it
statically at build time.  This requires a new build time helper macro to
declare a command and the completion handler at the same time.  Then we
convert the env related funcs over to this.

This gives an opportunity to also unify the U_BOOT_CMD macros.
Signed-off-by: NMike Frysinger <vapier@gentoo.org>