On some systems, RAM starts at address 0. If the user executes itest
against address 0 on such a system, it will call map_physmem(0, ...)
which will return 0 back; mapping only changes the address on sandbox.
This causes itest to believe map_physmem() has failed, and hence fails
the comparison.

Fix itest so that it allows map_physmem() to return 0 /if/ the orignal
address passed to it was also 0.

This fixes "tegra-uboot-flasher flash" on Tegra20.

This has the disadvantage that on sandbox, failed mapping attempts for
address 0 are not detected. Instead, should the code only call
map_physmem() on sandbox? Or, should map_physmem() return its error status
some other way. Or, should the special case only be allowed on systems
where the base of RAM is 0 somehow?

Fixes: 7861204c

 ("itest: make memory access work under sandbox")
Signed-off-by: Stephen Warren <swarren@nvidia.com>