ffmpeg: disable cpu autodetection

Just pass the correct -mcpu= configure switch (fallback to generic).

Avoids warnings like:
| warning: switch -mcpu=cortex-a15 conflicts with -march=armv7-a switch

(From OE-Core rev: 68aad8d179161c5fec75bf311060d829e4346c9f)

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-multimedia/ffmpeg/ffmpeg_3.2.2.bb

@@ -66,6 +66,12 @@ PACKAGECONFIG[xv] = "--enable-outdev=xv,--disable-outdev=xv,libxv"
 # Check codecs that require --enable-nonfree
 USE_NONFREE = "${@bb.utils.contains_any('PACKAGECONFIG', [ 'openssl' ], 'yes', '', d)}"
 
+def cpu(d):
+    for arg in (d.getVar('TUNE_CCARGS') or '').split():
+        if arg.startswith('-mcpu='):
+            return arg[6:]
+    return 'generic'
+
 EXTRA_OECONF = " \
     --disable-stripping \
     --enable-pic \
@@ -90,6 +96,7 @@ EXTRA_OECONF = " \
     --shlibdir=${libdir} \
     --datadir=${datadir}/ffmpeg \
     ${@bb.utils.contains('AVAILTUNES', 'mips32r2', '', '--disable-mipsdsp --disable-mipsdspr2', d)} \
+    --cpu=${@cpu(d)} \
 "
 
 EXTRA_OECONF_append_linux-gnux32 = " --disable-asm"