The common subdir sets up a cconfig.h file to hold checks for the common
code. In practice, most files still end up using config.h instead which
just leads to confusion.
Merge all the configure checks that went into cconfig.h into SIM_AC_COMMON
so we can drop the cconfig.h file altogether. Now there is only a single
config.h file like normal.
Rather than manually include tconfig.h when we think we'll need it (which
is error prone as it can define symbols we expect from config.h), have it
be included directly by config.h. Since we know we have to include that
header everywhere already, this will make sure tconfig.h isn't missed.
It should also be fine as tconfig.h is supposed to be simple and only set
up a few core defines for the target.
This allows us to stop symlinking it in place all the time and just use
it straight out of the respective source directory.
