loader: lua: test that /boot exists first

Otherwise on fs like tftp where no directory listing is possible we fail
on the .dir method.

Reviewed by: imp, kevans
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33414

(cherry picked from commit c4dc907254a4275d8f3ecd9a6b615c6e2a2438e9)