linux-imx

kbuild: exploit parallel building for CONFIG_HEADERS_CHECK

When CONFIG_HEADERS_CHECK is enabled, the headers_check is executed
as a serialized task in the vmlinux recipe.

Make it independent of vmlinux so that parallel building can process
the headers_check and other build targets simultaneously.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>