common: Add a noisy assert()

Some U-Boot code uses BUG_ON() and WARN_ON() macros. These use __FILE__
which can include quite a large path, depending on how U-Boot is built.

The existing assert() is only checked if DEBUG is enabled. Add a new one
which is always checked, and prints a (smaller) error in that case.

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