efi loader: disallow user to configure staging area size less than default

We need to round it up to 2M, for instance. Having staging area too small
might cause the first resize to use negative size for memmove()/memcpy(),
which kills loader.

Tested by: Harry Schmalzbauer <freebsd@omnilan.de>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week