stubdom: fix compile errors in grub

stubdom: fix compile errors in grub

Building xen.rpm in SLES11 started to fail due to these compiler
warnings:

[ 1436s] ../grub-upstream/netboot/fsys_tftp.c:213: warning: operation on 'block' may be undefined
[ 1437s] ../grub-upstream/netboot/main.c:444: warning: operation on 'block' may be undefined

[ 1234s] E: xen sequence-point ../grub-upstream/netboot/fsys_tftp.c:213
[ 1234s] E: xen sequence-point ../grub-upstream/netboot/main.c:444

The reason for this is that the assignment is done twice:
 tp.u.ack.block = ((uint16_t)( (((uint16_t)((block = prevblock)) & (uint16_t)0x00ffU) << 8) | (((uint16_t)((block = prevblock)) & (uint16_t)0xff00U) >> 8)));

Fix this package build error by adding another patch for grub, which
moves the assignment out of the macro usage.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>