meson: build: Install systemd unit files

Tested: Verified the expected systemd service files were installed
        with -Dhost-bios-upgrade=enabled and -Dbmc-layout=ubi

Change-Id: I1a46dc3916da208ded9c07af60d7e6856d1fa0ee
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
2 files changed