Rather than having code to re-define bcd2bin() for the LinuxKPI
make sure libkern.h is always included before the LinuxKPI version.
Then only re-define our local LinuxKPI implementation. 
From the argument truncating wrapper call the libkern version.
If we change our libkern implementation in the future we can save
us the remainder of the hassle.  Given I need this to MFC,
which I am not sure we can with libkern, commit this intermediate
Suggested by: Johannes Berg (johannes sipsolutions.net) 
Suggested by: ian 
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC with: 548ada00e54a9e7745d041b1ec7f68f3bd493365