Define the ffs() function in the USB bootloader's global and
independent header file.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275506 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Optimise bit searching loop by using the ffs() function.
Make some related bit shifts unsigned while at it.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@275507 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f