Rework incorrect use of assert() and panic() in codebase

Assert a valid security state using the macro sec_state_is_valid().
Replace assert() with panic() in those cases that might arise
because of runtime errors and not programming errors.
Replace panic() with assert() in those cases that might arise
because of programming errors.

Fixes ARM-software/tf-issues#96

Change-Id: I51e9ef0439fd5ff5e0edfef49050b69804bf14d5
Merge pull request #177 from jcastillo-arm/jc/tf-issues/096

Rework incorrect use of assert() and panic() in codebase