Makefile: Rename INSTALL_CORE_FILES => INSTALL_CORE_SCRIPTS
Makefile: Rename INSTALL_CORE_FILES => INSTALL_CORE_SCRIPTS

These are all executables. Also remove unnecessary directory creation
now that all these files are in the same directory (since we removed
`stbt-batch.d` subdirectory).

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester