xen: arm: avoid unnecessary additional mappings in boot page tables.

xen: arm: avoid unnecessary additional mappings in boot page tables.

If the identity map is created at one level then avoid creating
entries further down the boot page tables, since these will be aliases at
strange virtual address.

For example consider an arm32 system (for simplicity) with Xen loaded at
address 0x40402000. As a virtual address this corresponds to walking offsets 1,
2 and 2 at the first, second and third levels respectively.

When creating the identity map we will therefore create a 1GB super mapping at
0x40000000 for the identity map, which is the one we want to use.

However when considering the second level we will see the offset 2 and create a
2MB mapping in slot 2 of boot_second. Since boot_second is mapped in slot 0 of
boot_first this corresponds to an unwanted mapping from virtual address
0x00400000 to physical address 0x40400000.

We still do not handle the case where the load address is within the 2MB range
starting just after XEN_VIRT_START. This is not a regression but this patch
tries to provide a more useful diagnostic message. We do handle loading at
exactly XEN_VIRT_START.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>