Add assert() for debug assertions

assert() is like BUG_ON() but compiles to nothing unless DEBUG is defined.
This is useful when a condition is an error but a board reset is unlikely
to fix it, so it is better to soldier on in hope. Assertion failures should
be caught during development/test.

It turns out that assert() is defined separately in a few places in U-Boot
with various meanings. This patch cleans up some of these.

Build errors exposed by this change (and defining DEBUG) are also fixed in
this patch.

Signed-off-by: Simon Glass <sjg@chromium.org>