kernel-module-subpackage: Fix expansion with -b parameter (bsc#1208179).
When -b is specified the script is prefixed with KMP_NEEDS_MKINITRD=1
which sets the variable for a simple command.
However, the script is no longer a simple command. Export the variable
instead.
Signen-off-by: Michal Suchanek <msuchanek@suse.de>