SW: add efibootmgr to GRMLBASE

SW: add efibootmgr to GRMLBASE

The grub-efi-amd64-bin package in Debian/stretch used to have efibootmgr in
its Depends:

| Package: grub-efi-amd64-bin
| Source: grub2
| Version: 2.02~beta3-5+deb9u1
| [...]
| Depends: grub-common (= 2.02~beta3-5+deb9u1), efibootmgr

... while the grub-efi-amd64-bin package in Debian buster and newer has
efibootmgr in its Recommends:

| Package: grub-efi-amd64-bin
| Source: grub2
| Version: 2.02+dfsg1-20
| [...]
| Depends: grub-common (= 2.02+dfsg1-20)
| Recommends: grub-efi-amd64-signed, efibootmgr

Therefore we're no longer pulling in efibootmgr via grub-efi-amd64-bin, while
it's certainly important to have it available, so add it to GRMLBASE.

Development sponsored by Sipwise GmbH, recorded as
TT#63652 in customers' ticket system.