Restore the definition of EFI_STAGING_SIZE
The definition can be overridden by users, and before f75caed644a5c it
was in MBs. Make the symbol' unit MB, to be compatible with users
customizations.
Reported and tested by: Harry Schmalzbauer <freebsd@omnilan.de>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week