s390/tty3270: add support for background color

s390/tty3270: add support for background color

3270 terminals support 8 background colors. Add the code to utilize
them. Unfortunately the line erase code need to be adjusted: Without
background colors, it was sufficient to just set the line length to
zero. With background colors, we need to put spaces with the correct
background color.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>