scripts: qemustart: support CONFIG_BINARY_FOLDER

If CONFIG_BINARY_FOLDER is set in .config, use that instead of "bin" as
the bindir.

That allows to set that config and easily run e.g.
`./scripts/qemustart armvirt 32`.

Signed-off-by: Andre Heider <a.heider@gmail.com>

scripts/qemustart

@@ -158,7 +158,8 @@ parse_args() {
 		return 1
 	}
 	[ -n "$o_subtarget" ] || o_subtarget="generic"
-	o_bindir="bin/targets/$o_target/$o_subtarget"
+	eval "$(grep ^CONFIG_BINARY_FOLDER= .config 2>/dev/null)"
+	o_bindir="${CONFIG_BINARY_FOLDER:-bin}/targets/$o_target/$o_subtarget"
 }
 
 start_qemu_armvirt() {