kboot: Mark the EFI specific parts of bootinfo.c

bootinfo.c is about to be shared with kboot since they create
substantially similar environments / metadata tagging / etc. Tag this
with #ifdef EFI for the moment until the proper abstracting out can
happen.

Sponsored by: Netflix

(cherry picked from commit b11aebff4d4a301bfeac516b5be45080c8a11142)