multiboot2: Avoid first 0x500 bytes

In some cases, add_buffer will actually try to allocate the buffer
at 0x0, which may not be acceptable by some kernels. Let's avoid
the first 0x500 bytes so we don't screw up the IVT and BDA.

Signed-off-by: Zhaofeng Li <hello@zhaofeng.li>
Signed-off-by: Simon Horman <horms@verge.net.au>