app: aboot: Add functions to validate userkey

Add functions to store, erase and get userkey which
is required for validation of AVB2.0.

Change-Id: I989df64e40865b508c5d32b601e6da2c7009fc9b
diff --git a/include/platform.h b/include/platform.h
index f6f739f..bce5970 100644
--- a/include/platform.h
+++ b/include/platform.h
@@ -97,4 +97,7 @@
 uint32_t get_page_size();
 int read_rollback_index(uint32_t loc, uint64_t *roll_back_index);
 int write_rollback_index(uint32_t loc, uint64_t roll_back_index);
+int get_userkey(uint8_t **user_key, uint32_t *user_key_size);
+int erase_userkey();
+int store_userkey(uint8_t *user_key, uint32_t user_key_size);
 #endif