refactor(build): renaming BL2_AT_EL3 to RESET_TO_BL2

Along with e09da7be3fe5f7a59f16abc6008a8a97c3e1c247 that introduces a
breaking refactoring change we have to rename the build option in the CI
scripts.
Renaming BL2_AT_EL3 to RESET_TO_BL2.

Signed-off-by: Maksims Svecovs <maksims.svecovs@arm.com>
Change-Id: I57fe6c8e289a71d8d3cea3a6d71760d7df43112e
diff --git a/tf_config/fvp-bl2-el3 b/tf_config/fvp-bl2-el3
index 011094b..79f2539 100644
--- a/tf_config/fvp-bl2-el3
+++ b/tf_config/fvp-bl2-el3
@@ -1,3 +1,3 @@
-BL2_AT_EL3=1
+RESET_TO_BL2=1
 CROSS_COMPILE=aarch64-none-elf-
 PLAT=fvp