[mythtv] New configure script oddities

[Anduin Withers]

If one is foolish enough to pass --cpu=i686 in to the configure script it
isn't really honored. The problem is that through normal auto-detection $cpu
would be set to x86, in the case where it is overridden this transformation
doesn't occur (that chunk of script is before the option processing chunk)
this leads to unpleasant things like TARGET_ARCH_X86=yes not being output to
config.mak.

-- 
Anduin Withers