ffmpeg: fix bfin build

A gcc bug is triggered when Os/O2/O3 is used to compile the function.
Just force O1 for the problematic function for Blackfin.
See gcc bug report:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77311

Fixes:
  http://autobuild.buildroot.net/results/1118d568959006b7f973761884fd3f4b63be19b1/

Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/ffmpeg/0013-bfin-disable-optimization.patch

@@ -0,0 +1,26 @@
+Fix bfin compile error
+
+See gcc bug report:
+https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77311
+
+Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
+
+diff -Nur ffmpeg-2.8.7.orig/libavcodec/hevcdsp_template.c ffmpeg-2.8.7/libavcodec/hevcdsp_template.c
+--- ffmpeg-2.8.7.orig/libavcodec/hevcdsp_template.c	2016-03-29 04:25:16.000000000 +0200
++++ ffmpeg-2.8.7/libavcodec/hevcdsp_template.c	2016-08-12 21:32:36.728178969 +0200
+@@ -1517,7 +1517,14 @@
+ #define TQ2 pix[2  * xstride + 3 * ystride]
+ #define TQ3 pix[3  * xstride + 3 * ystride]
+ 
+-static void FUNC(hevc_loop_filter_luma)(uint8_t *_pix,
++// Blackfin gcc 6.1.x fails with
++// unable to find a register to spill in class CCREGS
++#if defined(__bfin__)
++#define disable_opt __attribute__ ((optimize("O1")))
++#else
++#define disable_opt 
++#endif
++static void disable_opt FUNC(hevc_loop_filter_luma)(uint8_t *_pix,
+                                         ptrdiff_t _xstride, ptrdiff_t _ystride,
+                                         int beta, int *_tc,
+                                         uint8_t *_no_p, uint8_t *_no_q)