Fix a problem which made loader(8) load non-kld files twice.

For example, without this patch, the following three lines
in /boot/loader.conf would result in /boot/root.img being preloaded
twice, and two md(4) devices - md0 and md1 - being created.

initmd_load="YES"
initmd_type="md_image"
initmd_name="/boot/root.img"

Reviewed by: marcel@
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D3204