Unmask SError interrupt and clear SCR_EL3.EA bit

This patch disables routing of external aborts from lower exception levels to
EL3 and ensures that a SError interrupt generated as a result of execution in
EL3 is taken locally instead of a lower exception level.

The SError interrupt is enabled in the TSP code only when the operation has not
been directly initiated by the normal world. This is to prevent the possibility
of an asynchronous external abort which originated in normal world from being
taken when execution is in S-EL1.

Fixes ARM-software/tf-issues#153

Change-Id: I157b996c75996d12fd86d27e98bc73dd8bce6cd5
diff --git a/bl1/aarch64/bl1_entrypoint.S b/bl1/aarch64/bl1_entrypoint.S
index dd7d78f..e7f92c7 100644
--- a/bl1/aarch64/bl1_entrypoint.S
+++ b/bl1/aarch64/bl1_entrypoint.S
@@ -76,6 +76,14 @@
 	 */
 	adr	x0, bl1_exceptions
 	msr	vbar_el3, x0
+	isb
+
+	/* ---------------------------------------------
+	 * Enable the SError interrupt now that the
+	 * exception vectors have been setup.
+	 * ---------------------------------------------
+	 */
+	msr	daifclr, #DAIF_ABT_BIT
 
 	/* ---------------------------------------------------------------------
 	 * The initial state of the Architectural feature trap register