Since ubldr doesn't necessarily load a kernel at the physical address in the
elf headers, mask out the high nibble of that address.  This effectly makes
the entry point the offset from the load address, and it gets adjusted for
the actual load address before jumping to it.

Masking the high nibble makes assumptions about memory layout that are true
for all the arm platforms we support right now, but it makes me uneasy.
This needs to be revisited.