In order to enable compile time differences in HAB interaction, we should
split out the definition of the base address of the HAB API.

Some version of the i.MX series have different offsets from the BootROM
base for the HAB callback table.

This patch defines the header into which we will define the i.MX7 specific
offset. The offset of the i.MX7 function-callback table is simultaneously
defined.

Once done, we can latch a set of common function pointer locations from the
offset given here and if necessary change the offset for different
processors without any other code-change.

For now all we support is i.MX7 so the only offset being defined is that
for the i.MX7.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Ryan Harkin <ryan.harkin@linaro.org>