scripts: logging/dictionary: extract actual strings for db

This changes the database generation to actually extracting
individual strings instead of stuffing the whole binary sections
into the database. This allows the generation script to be
extended to accommodate more output formats.

Note that if CONFIG_LOG2_FMT_SECTION is enabled, the format
strings are in log_strings_sections, and also have associated
debug symbols in DWARF. So there is no need to manually
extract them.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>