kbuild: Show marked Kconfig fragments in "help"

Currently the Kconfig fragments in kernel/configs and arch/*/configs
that aren't used internally aren't discoverable through "make help",
which consists of hard-coded lists of config fragments. Instead, list
all the fragment targets that have a "# Help: " comment prefix so the
targets can be generated dynamically.

Add logic to the Makefile to search for and display the fragment and
comment. Add comments to fragments that are intended to be direct targets.

Signed-off-by: Kees Cook <keescook@chromium.org>
Co-developed-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

powerpc: Add config fragment for disabling -Werror

This makes it easy to disable building with -Werror:

$ make defconfig
$ grep WERROR .config
# CONFIG_PPC_DISABLE_WERROR is not set
CONFIG_PPC_WERROR=y

$ make disable-werror.config
GEN Makefile
Using .config as base
Merging arch/powerpc/configs/disable-werror.config
Value of CONFIG_PPC_DISABLE_WERROR is redefined by fragment arch/powerpc/configs/disable-werror.config:
Previous value: # CONFIG_PPC_DISABLE_WERROR is not set
New value: CONFIG_PPC_DISABLE_WERROR=y
...

$ grep WERROR .config
CONFIG_PPC_DISABLE_WERROR=y

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201023040002.3313371-1-mpe@ellerman.id.au