Add a way to control the level of checks in the code

Add a new ASSUME_MASK option, which allows for some control over the
checks used in libfdt. With all assumptions enabled, libfdt assumes that
the input data and parameters are all correct and that internal errors
cannot happen.

By default no assumptions are made and all checks are enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Message-Id: <20200220214557.176528-3-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>