Commit cf0c3e68aa81 "kbuild: fix asm-offset generation to work with clang"
changed the macros used by devicetable-offsets.c. Copy the new sed code
from upstream scripts/Makefile.lib.
By passing both -C and -f options to make, we can separate makefiles
from the output directory without adding prefixes to all targets.
We can also reduce the 'clean' commands to little more than 'rm -rf'.