test: Populate/cleanup /etc "manually" via Makefile/tests

For now, do not mount /etc as tmpfs, but instead create the dir when
building the rootfs via test/test_root/Makefile.

Signed-off-by: Magnus Malm <magnusmalm@gmail.com>

test/test_root/Makefile

@@ -35,7 +35,8 @@ dirs = ./dev \
        ./sbin \
        ./sys \
        ./test_assets \
-       ./tmp
+       ./tmp \
+       ./etc
 
 _libs_src = $(shell ldd $(FINITBIN) | grep -Eo '/[^ ]+')
 libs = $(foreach path,$(_libs_src),.$(path))