Revert "Fix building tools alone with host compiler"

This reverts commit bbc6353c.
It breaks building on many systems:
...
.../common/env_embedded.c:28:20: fatal error: config.h: No such file or directory
compilation terminated.
.../common/image.c:27:20: fatal error: common.h: No such file or directory
compilation terminated.
.../lib/crc32.c:12:20: fatal error: common.h: No such file or directory
compilation terminated.
.../lib/md5.c:28:22: fatal error: compiler.h: No such file or directory
compilation terminated.
.../lib/sha1.c:33:20: fatal error: common.h: No such file or directory
compilation terminated.