package/mender-grubenv: fix installation with genimage.sh script

package/mender-grubenv: fix installation with genimage.sh script

mender-grubenv currently has 3 problems that prevent an x86_64-efi image from
successfully being made with the genimage.sh script.

- mender-grubenv does not currently depend on Grub2.
  While Grub2 is not needed to build the mender-grubenv package, Grub2 needs
  to be built first for mender-grubenv to overwrite the default Grub2 files
  reliably.

- The MENDER_GRUBENV_ENV_DIR variable points to /boot/efi/EFI/BOOT instead of
  /boot/EFI/BOOT, which is where the Grub2 package installs the default files.
  This variable now points to the correct location.

- The Grub2 package installs images to $(BINARIES_DIR)/efi-part, which the
  mender-grubenv package currently does not do. As such; the default Grub2
  configuration file is used instead of the one provided by mender-grubenv.
  Adding a MENDER_GRUBENV_INSTALL_IMAGES_CMDS define in mender-grubenv.mk which
  copies the installed files from $(TARGET_DIR)/boot/EFI to
  $(BINARIES_DIR)/efi-part fixes this issue.

Signed-off-by: Adam Duskett <aduskett@greenlots.com>
[Thomas:
 - drop "runtime" on the depends on BR2_TARGET_GRUB2 since we now have
   a build-time dependency on it
 - explicitly copy the files installed by mender-grubenv in
   MENDER_GRUBENV_INSTALL_IMAGES_CMDS instead of blindly copying
   everything that is in $(TARGET_DIR)/boot/EFI]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>