unix/Makefile: coverage: Explicitly build "axtls" too.

"coverage" build uses different BUILD directory comparing to the normal
build. Previously, any build picked up libaxtls.a from normal build's
directory, but that was fixed recently. So, for each build, we must
build axtls explicitly.

This fixes Travis build in particular.

ports/unix/Makefile

@@ -249,7 +249,7 @@ coverage:
 	    -DMICROPY_UNIX_COVERAGE' \
 	    LDFLAGS_EXTRA='-fprofile-arcs -ftest-coverage' \
 	    FROZEN_DIR=coverage-frzstr FROZEN_MPY_DIR=coverage-frzmpy \
-	    BUILD=build-coverage PROG=micropython_coverage
+	    BUILD=build-coverage PROG=micropython_coverage axtls all
 
 coverage_test: coverage
 	$(eval DIRNAME=ports/$(notdir $(CURDIR)))