Handle correctly situation when the OUTPUT variable is not defined.
Before the following error was printed:
$ make
riscv32-elf-size .elf
make: riscv32-elf-size: Command not found
make: *** [post-build] Error 127

Now it prints out:
$ make
OUTPUT variable is mandatory
make: *** [check-def] Error 1

--Additionally check if variable TRTL_SW is defined.
Signed-off-by: Adam Wujek <adam.wujek@cern.ch>