meta-agl-bsp: Enable overlayfs

meta-agl-bsp: Enable overlayfs

Add kernel configuration fragment to enable overlayfs.  The initial
planned use is for enabling generating coverage results on target,
but it will also be useful for container runtimes.

Bug-AGL: SPEC-3589

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I4dd5ddd2da52eac5b273b01be51339e03786b845
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25336
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>
(cherry picked from commit f5b7b4a5c5ffcd51509c3ed57dfb08fdce6cf01c)
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/25328