package/mender-grubenv: fix modules list help and variable

The help was missing the regexp module.

The variable assignment was missing spaces around the equal sign.

Signed-off-by: Adam Duskett <aduskett@gmail.com>
[yann.morin.1998@free.fr: split to its own patch, fix assignment too]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Köry Maincent <kory.maincent@bootlin.com>