hs38_vdk: use overlay for specific features in "inittab"

This is the only way to instantiate getty on both serial port
(ttyS3) and LCD/keyboard (tty0).

And since we use custom "inittab" remove fs remount from it so
no need to extra option in Buildroot config.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>