sdcard: rpi-sdimg: fix mcopy issue with boot.scr

boot.scr can be picked up elsewhere and fail the mcopy into
the boot filesystem. Currently need to allow the overwriting
which requires the whole bbclass to be mostly cloned from
meta-raspberrypi

Change-Id: I2839c782ec9a96a1c822dc1369e7e72aea0872e4
Bug-AGL: SPEC-409
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9319
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>