refactor: improve readability of symbol table

Make the symbol table produced by the memory mapping script more
readable. Add a generic interface for interacting with ELF binaries.
This interface enables us to get symbols that provide some insights into
TF-A's memory usage.

Change-Id: I6646f817a1d38d6184b837b78039b7465a533c5c
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>