chelper: Fix check for failed code build

Commit 73b78af6 inadvertently removed the check for a successful gcc
compilation.  Add the check back in.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

scripts/spi_flash/fatfs_lib.py

@@ -73,7 +73,7 @@ def check_fatfs_build(printfunc=lambda o: o):
         else:
             cmd = "%s %s" % (chelper.GCC_CMD, chelper.COMPILE_ARGS)
         printfunc("Building FatFS shared library...")
-        os.system(cmd % (destlib, ' '.join(srcfiles)))
+        chelper.do_build_code(cmd % (destlib, ' '.join(srcfiles)))
         printfunc("Done\n")
     global fatfs_ffi_main, fatfs_ffi_lib
     ffi_main.cdef(FATFS_CDEFS)