weston-ini-conf: do not enable gRPC proxy by default

weston-ini-conf: do not enable gRPC proxy by default

Since building the agl-compositor gRPC proxy is only enabled when
using meta-agl-demo, referencing it in the default Weston
configurations when building meta-agl-core standalone is less than
ideal.  To avoid this, refactor the WESTON_FRAGMENTS variable to use
a WESTON_FRAGMENTS_BASE variable for the non-display fragments, and
drop "grpc-proxy" from it.  The bbappend in meta-agl-demo can now
just append it WESTON_FRAGMENTS_BASE to get the desired end result
for the demo images.

Bug-AGL: SPEC-5183

Change-Id: Ia351212023d1e426acc5be23a21b3de84660a024
Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/30006
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Tested-by: Jenkins Job builder account