Revert accidental removal of BL2 from help message

Commit 375f538a in Github accidentally removed the BL2 targets from the
Makefile help message. This patch reverts that change.

Change-Id: I825a9abe5b4ba0f15d02879dda1056912e2ad60c

Makefile

@@ -420,8 +420,9 @@ help:
 	@echo "PLAT is used to specify which platform you wish to build."
 	@echo ""
 	@echo "Supported Targets:"
-	@echo "  all            Build the BL1, BL31 binaries"
+	@echo "  all            Build the BL1, BL2 and BL31 binaries"
 	@echo "  bl1            Build the BL1 binary"
+	@echo "  bl2            Build the BL2 binary"
 	@echo "  bl31           Build the BL31 binary"
 	@echo "  checkcodebase  Check the coding style of the entire source tree"
 	@echo "  checkpatch     Check the coding style on changes in the current"