Kbuild: Fix cleanup of generated sources in tools

On 'make clean', generated C files in tools/env/ and tools/boot/ are
currently not removed, but they should.

Auto-generation for shared sources was first introduced with
ad80c4a3 ("kbuild, tools: generate wrapper C sources automatically
by Makefile"). Cleanup later regressed (see Fixes:), because shared
files were moved out of lib/ and common/, but 'clean-dirs := lib common'
was not adjusted accordingly. Further, the generated
tools/env/embedded.c became a sibling to project files, which prevents
directory-wise cleanup at all.

To solve it, we establishe tools/generated/ as the sole place for
generated sources. Wrappers are now generated as
tools/generated/<orig_dirname>/<orig_filename>, and 'make clean' can
remove tools/generated/ as a whole (Linux Makefile.asm-generic headers
are cleaned similarly). This way we don't have to maintain separate
clean-files or clean-dirs entries for each single added or moved wrapper
file.

Fixes: 0649cd0d ("Move environment files from common/ to env/")
Fixes: 19a91f24 ("Create a new boot/ directory")
Signed-off-by: Tobias Deiminger <tdmg@linutronix.de>
[trini: Correct mkfwupdate case]
Signed-off-by: Tom Rini <trini@konsulko.com>