Rename 51-grub to 51-bootloader

This patch is part of a another big patch that will implement a fallback
to Extlinux in case GRUB2 is not available in the system. The reason why
it's being splitted in two patches is because git is not being able to
recognize the file changes when the rename happens in the same commit.

Change-Id: Ic2027dfa057fd6d62b532203b7ff50c3c739bccb