When building sandbox with ccache, one would hit warnings such as:
    warning: 'struct mmsghdr' declared inside parameter list
on random files; a way to reproduce this issue is to build a simple
file doing just:
    #include <sys/socket.h>

    int main(void) {
        return 0;
    }

    gcc -Wall -P -c -o foo foo.c

But actually the -P flag is only useful when generating non-C files,
such as linker scripts in the case of barebox.  Removing the -P flag
from all the gcc invocations, except when generating .lds files makes
the warning go away.  It turns out that this is what
linux/scripts/Makefile.build also does nowadays.

Signed-off-by: Loïc Minier <loic.minier@linaro.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>