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/corstone1000-fpga-default b/tf_config/corstone1000-fpga-default
index df7899e..d0b8b23 100644
--- a/tf_config/corstone1000-fpga-default
+++ b/tf_config/corstone1000-fpga-default
@@ -1,5 +1,5 @@
 ARM_ROTPK_LOCATION=devel_rsa
-BL2_AT_EL3=1
+RESET_TO_BL2=1
 CROSS_COMPILE=aarch64-none-elf-
 ENABLE_PIE=1
 GENERATE_COT=1