lib/grub2: Port some to new code style
lib/grub2: Port some to new code style

I resisted trying to do anything invasive here like fd-relative porting as our
coverage is weak. But this was all straightforward porting to decl-after-stmt
style.

Closes: #1153
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree