MFC r355796-r355797, r355799: kbd: defaults for get_fkeystr/diag

MFC r355796-r355797, r355799: kbd: defaults for get_fkeystr/diag

The genkbd version of these remains exposed for stable branches, but
keyboard drivers that just want to use the defaults can simply not provide
their own. There shouldn't be any unset in the wild.

r355796:
kbd: provide default implementations of get_fkeystr/diag

Most keyboard drivers are using the genkbd implementations as it is;
formally use them for any that aren't set.

r355797:
chrome_kb: remove default get_fkeystr/diag implementations

This file was missed in r355796, but no harm would have come from this.

r355799:
kbd: patch linker set methods, too

This is needed after r355796. Some double-registration of kbd drivers needs
to be sorted out, then this sysinit will simply add these drivers into the
normal list and kill off any other bits in the driver that are aware of the
linker set, for simplicity.