scripts: size_report: alter how nodes are grouped together

This changes how the tree is generated. Symbols with no paths
are now grouped together instead of scattering throughout
the tree. If paths in ELF file are not all under ZEPHYR_BASE,
it will create additional node groups as 2nd level. This is
useful when not all source files are under ZEPHYR_BASE, and
provides a better indications of where they are.

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