Fix atkbdmap.h generation for sc/vt consoles

Keymap header files have historically been generated using the build
host's /usr/sbin/kbdcontrol and using the host's keymap files.

However, that introduces an issue when building a kernel to use vt(4)
on a system using sc(4), or vice versa: kbdcontrol searches for keymap
files in the /usr/share subdirectory appropriate for the host, not the
target.

With this change the build searches both the and sc keymap directories
from the source tree.

PR: 193865
Submitted by: Harald Schmalzbauer