iosbuild.sh: Add vpx_config.h and vpx_version.h to VPX.framework.
iosbuild.sh: Add vpx_config.h and vpx_version.h to VPX.framework.

- Rename build_targets to build_framework
- Add functions for creating the vpx_config shim and obtaining
  preproc symbols.

Change-Id: Ieca6938b9779077eefa26bf4cfee64286d1840b0

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