cmake: Show executable for memory report

Firstly, COMMENT does not work for ninja.
Therefore, change COMMENT to echo.

With the addition of sysbuild, several memory reports can be printed
for one build.
Because of this, it is useful to know which executable each memory
report is printed for, so adding full path to elf file.

Signed-off-by: Sigurd Hellesvik <sigurd.hellesvik@nordicsemi.no>