Revert "uImage: use abspath to get full path to mkimage"

This reverts commit 5e7fb57604921fef24878149c1bea396a1883c98.
Revert "Create proper make target for U-Boot mkimage and use it for kernel build"

This reverts commit f6be6c8c418ee68d0204c7b4bef5aaab861887ac.
Revert "TARGET_BOOTLOADER_IMAGE: fix u-boot binary file names"

This reverts commit 89bea7a55e62b0a5629de0bd6a1744e4db80e0b5.
Revert "U-Boot: copy various builds to u-boot.configname"

This reverts commit 95855ecb2a519521be660dfa63434d6dd66f2616.
Revert "BOOTLOADER: build and install u-boot.imx, not u-boot.bin"

This reverts commit e6b30c6207f84e634d0511847424c93313446517.