Added note and compile time check about big-endian issue

BUG:25017606
Change-Id: I8fda6d3edcd71bcee8021f0f1224b9d887beb2e8
Reviewed-on: https://weave-review.googlesource.com/1383
Reviewed-by: Vitaly Buka <vitalybuka@google.com>

libweave/README

diff --git a/libweave/README b/libweave/README
index a4875fd..b772553 100644
--- a/libweave/README
+++ b/libweave/README
@@ -115,3 +115,7 @@
   git push origin HEAD:refs/for/master
 
 Go to the url from the output of "push" and add reviewers.
+
+Known Issues
+------------
+* No big-endian support. Pairing fails on big-endian hardware.

libweave/src/privet/security_manager.cc

diff --git a/libweave/src/privet/security_manager.cc b/libweave/src/privet/security_manager.cc
index 8b1500c..1b4e3c5 100644
--- a/libweave/src/privet/security_manager.cc
+++ b/libweave/src/privet/security_manager.cc
@@ -24,6 +24,10 @@
 #include "src/string_utils.h"
 #include "third_party/chromium/crypto/p224_spake.h"
 
+#if !defined(ARCH_CPU_LITTLE_ENDIAN)
+#error "Big-endian is not supported. See b/25017606"
+#endif
+
 namespace weave {
 namespace privet {