kbuild: do not hide clang warnings during build with W=1

These lines originate in Linux;  it looks like Linux folks chose to
hide bunch of warnings reported by clang rather than fixing C code.

In U-boot, warnings are being fixed thanks to Jeroen's great efforts.
Let's stop suppressing clang warnings.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Jeroen Hofstee <jeroen@myspectrum.nl>
Series-cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Series-version: 2
Series-changes: 2
  - Rephrase commit summary and description for clarification
kbuild: remove "clobber" target

Now we stick to Linux Kernel's build system, so squash
"clobber" to "clean" target.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>