Updated srec extractor to only extract s-records that start with a header field

src/binwalk/config/extract.conf

@@ -42,7 +42,7 @@
 ^osx dmg:dmg:7z x '%e'
 ^lzo compressed data:lzo:lzop -f -d '%e'
 ^intel hex:hex:srec_cat '%e' -Intel -Output '%e.bin' -Binary
-^motorola s-record:srec:srec_cat '%e' -Motorola -Output '%e.bin' -Binary
+^motorola s-record.*header$:srec:srec_cat '%e' -Motorola -Output '%e.bin' -Binary
 
 # jar just does a better job of extracting zip files than either
 # unzip or 7z.