x86: Relocate boot trampoline to avoid BIOS conflicts.

x86: Relocate boot trampoline to avoid BIOS conflicts.

Fix booting through iSCSI protocol with Broadcom network cards.

These boards use the option ROM feature to implement the TCP/IP stack
protocol, and the iSCSI software initiator. The memory address
normally used by the PMM is 0x87000 which conflicts with the memory
allocation for Xen's trampoline routine, currently 0x88000.

Relocating down to 0x7c000 fixes the problem.

Signed-off-by: Fabio Guarneri <fabioguarneri@gmail.com>