weston-ini-conf.bb: Change from append to an inline edit

weston-ini-conf.bb: Change from append to an inline edit

We don't really support section entries with the same name, so rather
than appending a new key entry value under the same section, use sed to
edit the configuration file in place.

Fixes 6a8a33f99a66ae1

Bug-AGL: SPEC-4528
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: I12b982dd7e6dd8d11e5fb373c8cba0b1e762c485
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/28019
Tested-by: Jenkins Job builder account
ci-image-build: Jenkins Job builder account
ci-image-boot-test: Jenkins Job builder account
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>