toolchain-external wrapper: don't pass march/mcpu if mtune is on cmdline

Before commit 5715d2dcf48a39c16a3f3e41c97109613fed121d, the external
toolchain wrapper would not pass its own march/mcpu/mtune flags to the real
compiler if at least one of them was passed on the wrapper command-line.

The mentioned commit intended to remove the passing of an mtune parameter
coming from Buildroot, which was always empty after some other refactoring,
but the changes have the side-effect that march/mcpu is now also passed when
mtune is already given on the command-line. In that case, only mtune should
be passed to the real compiler.

Restore part of the original toolchain wrapper code to check the presence of
mtune on the command-line.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/toolchain-external/ext-toolchain-wrapper.c

@@ -177,11 +177,12 @@ int main(int argc, char **argv)
 
 #if defined(BR_ARCH) || \
     defined(BR_CPU)
-	/* Add our -march/cpu/abi flags, but only if none are
-	 * already specified on the commandline
+	/* Add our -march/cpu flags, but only if none of
+	 * -march/mtune/mcpu are already specified on the commandline
 	 */
 	for (i = 1; i < argc; i++) {
 		if (!strncmp(argv[i], "-march=", strlen("-march=")) ||
+		    !strncmp(argv[i], "-mtune=", strlen("-mtune=")) ||
 		    !strncmp(argv[i], "-mcpu=",  strlen("-mcpu=" )))
 			break;
 	}