build: Fix error_resilience code dependencies
mpegvideo: Conditionally build error_resilience bits

This breaks the dependency of mpegvideo on error_resilience allowing
compilation of components that depend on the former w/o the latter.
Add missing error_resilience includes to files that use ER
configure: Add missing h264chroma dependencies to vp5, vp6
build: cosmetics: Group hw accelerator Makefile entries together
configure: Add missing videodsp dependencies to some decoders
avstring: Fix isxdigit to not accept non-hex characters

Signed-off-by: Martin Storsjö <martin@martin.st>