Refactor distro build support

Create dedicated functions for each task while modifying distro image
with custom kernel and initrd.

Also, change the directory name for uefi/distro artifacts to match it
with ci tests.

This refactoring is expected to be useful while adding support for the
new distro boot support in future with minimal modification.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>