kernel: add a workaround to rebuild vdso.so.dbg after genvdso

Currently the build fails with

  'arch/mips/vdso/vdso.so.dbg' already contains a '.MIPS.abiflags' section

if the file already exists from a prior build.

Add a makefile rule to force the rebuild of vdso.so.dbg if genvdso has
has been changed to workaround the failure.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

target/linux/generic/patches-4.4/207-mips-vdso-dbg-rebuild-after-genvdso.patch

+--- a/arch/mips/vdso/Makefile
++++ b/arch/mips/vdso/Makefile
+@@ -75,7 +75,7 @@ $(obj-vdso): KBUILD_AFLAGS := $(aflags-v
+ 
+ $(obj)/vdso.lds: KBUILD_CPPFLAGS := $(native-abi)
+ 
+-$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) FORCE
++$(obj)/vdso.so.dbg: $(obj)/vdso.lds $(obj-vdso) $(obj)/genvdso FORCE
+ 	$(call if_changed,vdsold)
+ 
+ $(obj)/vdso-image.c: $(obj)/vdso.so.dbg $(obj)/genvdso FORCE