multiboot: Fix bss segment support

multiboot: Fix bss segment support

Multiboot images can specify a bss segment. The boot loader must clear
the memory of the bss and ensure that no modules or structures are
allocated inside it. Several fields are provided in the Multiboot
header that were previously not used properly. The header is now used
to determine how much data should be read from the image and how much
memory should be reserved to the bss segment.

Signed-off-by: Göran Weinholt <goran@weinholt.se>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>