[WIC][IMX8] wks: fix overlap of imx-boot and boot partitions

The u-boot env offset in the imx-boot partition is 4MiB, so set the boot partition alignment to 8MiB to fix the overlap.

• reference: https://github.com/Freescale/meta-freescale/commit/6c34107a9a48b962d2fd50eb4a032fae12b844f3