s390-ccw: read stage2 boot loader data to find menu

s390-ccw: read stage2 boot loader data to find menu

Read the stage2 boot loader data block-by-block. We scan the
current block for the string "zIPL" to detect the start of the
boot menu banner. We then load the adjacent blocks (previous
block and next block) to account for the possibility of menu
data spanning multiple blocks.

Signed-off-by: Collin L. Walling <walling@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>