bootspec: use chase_symlinks() where appropriate

bootspec: use chase_symlinks() where appropriate

Let's better be safe than sorry, and follow symlinks in ESP + XBOOTLDR
via chase_symlinks() relative to the root of these files systems. This is
not a big issue given that ESP/XBOOTLDR are supposed to be VFAT where
symlinks don't exist, but the specs don't strictly require this, and by
loading efifs drivers in uefi it would be possible to support symlinks,
but we'd rather not have that here.

No real change in behaviour, just stricter rules to make me sleep better
at night.