BSPs: Do not use := for AGL_DEFAULT_IMAGE_FSTYPES

BSPs: Do not use := for AGL_DEFAULT_IMAGE_FSTYPES

An OpenEmbedded best practice would be to use '=' here to allow users to
append to it later on.

Bug-AGL: SPEC-899
Change-Id: I5a664df7c5eb8d71f0ab6d0723292f3a187fe752
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11229
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>
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>