meta-agl-bsp: imx6qsabrelite: Switch to WIC image

To produce a bootable WIC image, we needs to make some
changes in 6x_bootscript for compatibility with WIC and
SDCARD images.
Because in case of WIC, we have one partition, and the kernel
files are located in /boot directory.
In case of SDCARD image, kernel files are in root directory of
first partition.

Bug-AGL: SPEC-899

Change-Id: Ie4b26a703cc468ff8c8405e957aacc2973ee289a
Signed-off-by: Mihail Grigorov <michael.grigorov@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/11571
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>
Reviewed-by: Matt Porter <mporter@konsulko.com>