Auto-generate configuration flags.

This modifies import_openssl.sh to parse the configured Makefile
and extract the appropriate compiler flags that were currently
defined manually in android-config.mk

- Modifies openssl.config to add missing configure options to
  ensure the final result is the same than before the patch.
  This also updates crypto/opensslconf.h.

- The generated output is stored in build-config.mk which content
  directly comes from the OpenSSL Makefile.

- android-config.mk is still used to define LOCAL_CFLAGS from
  the definitions in build-config.mk, as well as perform minimal
  extra filtering.

- Remove the section in README.android about manually changing
  android-config.mk.

Change-Id: I5275de69a817aa7c9880ea48e5d6a8ac1652a1e4