scripts: python3: replace exec statement with builtin

Used exec() builtin instead of 'exec' statement as
this statement doesn't exist in python 3.

(From meta-yocto rev: ceb6c6fac212c5e1e76bf58b793dc2b9420b4a66)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/bsp/engine.py

@@ -1362,7 +1362,7 @@ def run_program_lines(linelist, codedump):
         of = open("bspgen.out", "w")
         of.write(buf)
         of.close()
-    exec buf
+    exec(buf)
 
 
 def gen_target(files, context = None):