stand/efi: Better variable name

sanity_check_currdev returns true if it found a kernel or a sane loader
config file. A better name for this would be 'bootable' rather than 'rv'
which connotes in other places an errno value or similar.

Sponsored by: Netflix

(cherry picked from commit 23ed2a38c200c93414de20d20f7508f621acecd1)