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.h b/recovery.h
index f050549..08bacbf 100644
--- a/recovery.h
+++ b/recovery.h
@@ -22,3 +22,5 @@
 #include "recovery_ui/device.h"
 
 Device::BuiltinAction start_recovery(Device* device, const std::vector<std::string>& args);
+
+std::string get_build_type();