Allow -nogfx boards to start the homescreen

Allow -nogfx boards to start the homescreen

This allows to produce working images without restricted licenses.
Software rendering will be slow and screen updates take a while.

v2: adapted to use WESTON_USE_PIXMAN

Bug-AGL: SPEC-3774
Signed-off-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
Change-Id: Iaea1985da48985dfe223471a86b04d8921cc8d7a
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25933
Reviewed-by: Scott Murray <scott.murray@konsulko.com>
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>