vga: Change the arguments of vga_write_text to support extended ASCII

VGA defined the extended ASCII set based on CP437, but the function
vga_write_text() accepts a signed char array.

This will cause unnecessary confusion that if we want to print u with
umlaut (code=129 in CP437), we need to explicitly cast it to -127 in
signed char.

Since we still want to leverage the built-in string utilities
which only accepts const char*, we still need to cast it to signed char
while processing, and cast it back to unsigned once we write into the
frame buffer.

BRANCH=brya
BUG=b:264666392
TEST=emerge-brya coreboot chromeos-bootimage

Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Change-Id: If555bbc05f40ce3f02339c0468afff6dda8b7ded
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73099
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tarun Tuli <taruntuli@google.com>

src/include/pc80/vga.h

@@ -33,6 +33,7 @@ void vga_line_write(unsigned int line, const char *string);
  * vga_write_text() writes a line of text aligned left/center/right
  * horizontally on the screen (i.e. enum VGA_TEXT_ALIGNMENT)
  */
-void vga_write_text(enum VGA_TEXT_ALIGNMENT alignment, unsigned int line, const char *string);
+void vga_write_text(enum VGA_TEXT_ALIGNMENT alignment, unsigned int line,
+		    const unsigned char *ustring);
 
 #endif /* VGA_H */