LinuxKPI: add bcd.h

Add bcd2bin() in bcd.h.

Libkern does provide a bcd2bin() which cannot be used cirectly leaving
us with a conflict (see comment in file).
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].

Suggested by: Johannes Berg (johannes sipsolutions.net) [1]
Suggested by: ian [2]
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 548ada00e54a9e7745d041b1ec7f68f3bd493365)
(cherry picked from commit ae2268efd5f8aaa6aba27dfc4e34b3225f211901)