Enable usrmerge

Enable usrmerge

An increasing number of advanced systemd features with respect to
immutable systems and sandboxing are effectively dependent on the
usrmerge distro feature, so it seems reasonable to enable it since
upstream AGL currently requires systemd usage.  See SPEC-4352 for
more detailed discussion of rationale.

Changes:
- Add usrmerge to AGL_DEFAULT_DISTRO_FEATURES in poky-agl.conf.
- Add workarounds via bbappends in meta-agl-bsp for the assumption
  of using /lib instead of ${nonarch_base_libdir} in various
  meta-rcar-gen3 kernel module recipes.  This will be followed up
  with upstream to hopefully fix the recipes.
- Replaced some explicit /sbin usage with ${sbindir} in the
  initramfs-netboot recipe used by the agl-netboot feature to avoid
  packaging errors.

Bug-AGL: SPEC-4352

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ibc0ae1e2628aca6474401e0852bbb97085f4ecfa
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/27437
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account