os/Makefile.unix: use -f option to delete output bin folder
os/Makefile.unix: use -f option to delete output bin folder

When loadable build configuration is built, the user folder under output/bin is newly created and
it should be deleted with clean command. But DELFILE definition can do only files, can't do folders.
It causes the error as below:

rm: cannot remove '../build/output/bin/user': Is a directory
Makefile.unix:629: recipe for target 'clean' failed
make: *** [clean] Error 1

This commit changes the command to delete output bin folder from DELFILE to DELDIR to resolve above.

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

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