This patch fixes a bug in the context management code that causes it to
ignore the HANDLE_EA_EL3_FIRST compile-time option and instead always
configure SCR_EL3 to force all external aborts to trap into EL3. The
code used #ifdef to read compile-time option declared with add_define in
the Makefile... however, those options are always defined, they're just
defined to either 0 or 1, so #if is the correct syntax to check for
them. Also update the documentation to match.

This bug has existed since the Nov 2017 commit 76454abf4 (AArch64:
Introduce External Abort handling), which changed the
HANDLE_EA_EL3_FIRST option to use add_define.

Change-Id: I7189f41d0daee78fa2fcf4066323e663e1e04d3d
Signed-off-by: Julius Werner <jwerner@chromium.org>