meta-agl-core: fix IMAGE_FSTYPES override

meta-agl-core: fix IMAGE_FSTYPES override

The anonymous python snippet in agl-image-minimal.inc to reset
IMAGE_FSTYPES needs to be before the inherit of core-image to
actually take effect when building.  This was not immediately
apparent from the variable expansion information visible in
"bitbake -e" output.  A comment explaining the snippet and this
requirement has been added for clarity.

Bug-AGL: SPEC-3741

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ife1dca4c3ff961a9f693631ec0dac498114d395d
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25859
Reviewed-by: Jan-Simon Moeller <jsmoeller@linuxfoundation.org>
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>