recovery: Allow bypassing signature verification on non-release builds

For non-release (userdebug, eng) builds, when signature verification
fails, ask the user whether they wish to install anyway.

[aleasto]
Rewritten to minimize the diff footprint for maintainability

Change-Id: I950ad455e6f698cabe348f0482eb64287cc88a08
diff --git a/recovery_main.cpp b/recovery_main.cpp
index 183950d..b15719e 100644
--- a/recovery_main.cpp
+++ b/recovery_main.cpp
@@ -77,7 +77,7 @@
   return "orange" == android::base::GetProperty("ro.boot.verifiedbootstate", "");
 }
 
-static std::string get_build_type() {
+std::string get_build_type() {
   return android::base::GetProperty("ro.build.type", "");
 }