- Turn Wstrict-prototypes into an error
- Add -Werror=implicit-int
- Use the standard form -Werror=... for implicit-function-declaration

This commit imports the following Linux commits. They should work for
barebox too.

80970472179a Makefile: enable -Werror=implicit-int and -Werror=strict-prototypes by default
eeb5687a7139 kbuild: add -Werror=strict-prototypes flag unconditionally
61a0902a06d6 kbuild: add -Werror=implicit-int flag unconditionally
b89f25ea7892 kbuild: use -Werror=implicit-... instead of -Werror-implicit-...

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>