kbuild: support W=c and W=e shorthands for Kconfig

KCONFIG_WARN_UNKNOWN_SYMBOLS=1 and KCONFIG_WERROR=1 are descriptive
and suitable in scripting, but typing them from the command line can
be tedious.

Associate them with KBUILD_EXTRA_WARN (and the W= shorthand).

Support a new letter 'c' to enable extra checks in Kconfig. You can
still manage compiler warnings (W=1) and Kconfig warnings (W=c)
independently.

Reuse the letter 'e' to turn Kconfig warnings into errors.

As usual, you can combine multiple letters in KCONFIG_EXTRA_WARN.

  $ KCONFIG_WARN_UNKNOWN_SYMBOLS=1 KCONFIG_WERROR=1 make defconfig

can be shortened to:

  $ KBUILD_EXTRA_WARN=ce make defconfig

or, even shorter:

  $ make W=ce defconfig

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>