Added new <environment> section to the existing <bootloadersettings>
section which allows to specify environment variables for setting
up an environment blob for the selected loader. With this commit
we add support for grub by using grub2-editenv. Other loaders
do not yet have an implementation or does not support environment
blobs. Settings will be ignored for unsupported loaders.
This Fixes#2922
Co-authored-by: Rhys Oxenham <rhys.oxenham@suse.com>