configs/freescale_imx6sololiteevk: Remove target

configs/freescale_imx6sololiteevk: Remove target

Currently this target fails to build:

Error: No BOOT_FROM tag in board/freescale/mx6slevk/imximage.cfg.cfgtmp
arch/arm/imx-common/Makefile:91: recipe for target 'u-boot-dtb.imx' failed

This problem has been already solved in upstream U-Boot, but not
on NXP version of U-Boot.

To avoid the maintainance burden of keeping two defconfigs for the
same board, remove freescale_imx6sololiteevk_defconfig in favor
of imx6slevk_defconfig, which uses U-Boot and kernel mainline.

Fixes: https://gitlab.com/buildroot.org/buildroot/-/jobs/66561693
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>