loader: instead of hw.vga.textmode, use screen.textmode

hw.vga.textmode is directing VT VGA backend to use text mode.

The default screen mode for BIOS loader is text, and default
screen mode for VT VGA backend is graphics (unless we are running on
hypervisor or hw.vga.textmode is set to 1). Using hw.vga.textmode
for loader does remove possibility to have graphical mode VT VGA with
text mode loader.

screen.textmode can have possible values "0" to disable text mode,
and "1" to set text mode.