mcuboot: Using eval instead of regex for macro_parser

The old method has some limitations, for example
when the macros using too many brackets.
Python's built in eval function can handle most cases.

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: Id98967fae8d039f4b7b61d6e772dbb233ee7f450