Add OPENSSL_SMALL.

Intel's P-256 code has very large tables and things like Chromium just
don't need that extra size. However, servers generally do so this change
adds an OPENSSL_SMALL define that currently just drops the 64-bit P-224
but will gate Intel's P-256 in the future too.

Change-Id: I2e55c6e06327fafabef9b96d875069d95c0eea81
Reviewed-on: https://boringssl-review.googlesource.com/6362
Reviewed-by: Adam Langley <alangley@gmail.com>
diff --git a/BUILDING.md b/BUILDING.md
index e87f2a6..7d4eac4 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -71,6 +71,10 @@
 shared library, define `BORINGSSL_SHARED_LIBRARY` in any code which `#include`s
 the BoringSSL headers.
 
+In order to serve environments where code-size is important as well as those
+where performance is the overriding concern, `OPENSSL_SMALL` can be defined to
+remove some code that is especially large.
+
 ### Building for Android
 
 It's possible to build BoringSSL with the Android NDK using CMake. This has