unix: When using separate obj output dirs, make -B is no longer relevant.

unix/Makefile

@@ -125,17 +125,14 @@ uninstall:
 
 # build synthetically fast interpreter for benchmarking
 fast:
-	@echo Make sure to run make -B
 	$(MAKE) COPT="-O2 -DNDEBUG -fno-crossjumping" CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_fast.h>"' BUILD=build-fast PROG=micropython_fast
 
 # build a minimal interpreter
 minimal:
-	@echo Make sure to run make -B
 	$(MAKE) COPT="-Os -DNDEBUG" CFLAGS_EXTRA='-DMP_CONFIGFILE="<mpconfigport_minimal.h>"' BUILD=build-minimal PROG=micropython_minimal MICROPY_PY_TIME=0 MICROPY_PY_TERMIOS=0 MICROPY_PY_SOCKET=0 MICROPY_PY_FFI=0
 
 # build an interpreter for coverage testing and do the testing
 coverage:
-	@echo Make sure to run make -B
 	$(MAKE) COPT="-O0" CFLAGS_EXTRA='-fprofile-arcs -ftest-coverage -Wdouble-promotion -Wformat -Wmissing-declarations -Wmissing-prototypes -Wold-style-definition -Wpointer-arith -Wredundant-decls -Wshadow -Wsign-compare -Wstrict-prototypes -Wuninitialized -Wunused-parameter' LDFLAGS_EXTRA='-fprofile-arcs -ftest-coverage' BUILD=build-coverage PROG=micropython_coverage
 
 coverage_test: coverage