LinuxKPI: make bcd.h use libkern

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. [1]

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. [2] Given I need this to MFC,
which I am not sure we can with libkern, commit this intermediate
step.

Suggested by: Johannes Berg (johannes sipsolutions.net) [1]
Suggested by: ian [2]
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC with: 548ada00e54a9e7745d041b1ec7f68f3bd493365
Differential Revision: https://reviews.freebsd.org/D32695