Have asserts call abort on error

The current KAI_ASSERT() function will exit the program using
`exit(EXIT_FAILURE)` which is unhelpful when running the program under a
debugger, as this will not trap the execution.

This change changes the call to `abort()` instead

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>