package/mender-grubenv: fix install for legacy BIOS or ARM U-Boot

Some last-minute changes were made when applying commits 7a68960b
(boot/grub2/Config.in: add symbols to represent legacy and EFI boot) and
4d5b2093 (package/mender-grubenv: fix grub module checks), and the
renaming of the BR2_TARGET_GRUB_LEGACY was not fully propagated.

This caused the path to the boot files to always be interpreted as being
the EFI one, and never the legacy one. In practice, that was not causing
any build failure, because the path was passed at build-time to
mender-grubenv, that would use it as the location where to install its
files, and finally as the location where our image-isntall commands
would look for them.

Still this is incorrect because it would not match where grub2 would
eventually end up lookig for its files at runtime...

To avoid further issue, drop the conditional block dedicated to setting
the path to the boot files, drop the intermediate variable, and move
setting the environment variable down into the existing conditional
block.

We do drop the intermediate variable, because there is no longer any
genericity needed: the installation commands are already duplicated for
the two cases anyway.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Adam Duskett <aduskett@gmail.com>
Cc: Köry Maincent <kory.maincent@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>