unix/Makefile: Update freedos target for change of USELECT config name.

unix/Makefile

@@ -229,7 +229,7 @@ freedos:
 	CC=i586-pc-msdosdjgpp-gcc \
 	STRIP=i586-pc-msdosdjgpp-strip \
 	SIZE=i586-pc-msdosdjgpp-size \
-	CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_freedos.h>" -DMICROPY_NLR_SETJMP -Dtgamma=gamma -DMICROPY_EMIT_X86=0 -DMICROPY_NO_ALLOCA=1 -DMICROPY_PY_USELECT=0' \
+	CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_freedos.h>" -DMICROPY_NLR_SETJMP -Dtgamma=gamma -DMICROPY_EMIT_X86=0 -DMICROPY_NO_ALLOCA=1 -DMICROPY_PY_USELECT_POSIX=0' \
 	BUILD=build-freedos \
 	PROG=micropython_freedos \
 	MICROPY_PY_SOCKET=0 \