Patch for RDI target code to allow user-specified devices

[Grant Edwards]

Hello,

The attached patch against 4.18 sources modifies the RDI target
unix comm code to allow users to specify any device they please
to be used for serial and/or parallel communication with the
target.

The current routines limit the user to a certain set of devices
(which never seems to include the one I want).  I've tripped
over this many times.  With the patch applied, the target
commands like

 target rdi s=/whatever/flipping/device/the/user/wants
 target rdi s=/serial/device/name,p=/parallel/device/name

Will accept whatever the user specifies (I think the path is
limited to 64 characters).  After all it's _my_ computer and I
do, in fact, know which serial port is hooked to the target!
The fact that the ARM Ltd. code won't believe me has always
been annoying...

Yes, I know, the patched code uses a different indenting style
that the rest of the file -- if this is a problem, feel free to
re-indent it however you please.  I simply can not easily grok
the existing style.

-- 
Grant Edwards
grante@visi.com