kernel: add CONFIG_KSU_DEBUG (#19)

* Kconfig: add KSU_DEBUG

* print alert on debug mode

* allow shell by default

* store signature to var on debug mode

* format

* export as module_param

* rename apk_sign to kernelsu

kernel/Makefile

@@ -1,6 +1,7 @@
 obj-y += ksu.o
 obj-y += allowlist.o
-obj-y += apk_sign.o
+kernelsu-objs := apk_sign.o
+obj-y += kernelsu.o
 obj-y += module_api.o
 obj-y += sucompat.o