Revision 337960

build: skip the database check when generating install media

There are several scripts and targets solely used to generate install
media, make sure DB_FROM_SRC is used in that case in order to prevent
checking the host database, which is irrelevant when generating
install binaries.

Sponsored by:		Citrix Systems R&D
PR:			230459
Reviewed by:		gjb
Differential revision:	https://reviews.freebsd.org/D16638