generate-config.sh: Support running from contexts where O is set

When invoked from a context like this...

   my-infix-spin$ make O=x-my-build x86_64_defconfig

...where the spin's Makefile would trigger the generation of the
defconfig based on some fragments, the value of O passed via the
environment would be ignored since the spin's Makefile typically
overrides it.

Therefore, pass it as a make variable instead, which takes precedence.

utils/generate-defconfig.sh

@@ -81,6 +81,6 @@ done
 TMPDIR=`mktemp -d`
 $MERGE_CONFIG -O "$TMPDIR" "$base" "$changes"
 
-O="$TMPDIR" make savedefconfig
+make O="$TMPDIR" savedefconfig
 mv "$TMPDIR"/defconfig "$output"
 rm -r "$TMPDIR"