Express dependency between core and bc targets

```
$ git grep '\.\./' | grep '^[A-z]\+\/[A-z]\+:'
...
core/Makefile:117:include/bif.h: ../bc/scripts/bif.tab
core/Makefile:129:gentab/exp_tab.erl: $(PREMODS) ../bc/scripts/bif.tab
core/Makefile:130:      scripts/exptab_gen preload ../bc/scripts/bif.tab $@
...
```
Call `test.img` - not `test` - target and express dep of `test` on `railing`

See last line below:
```
$ git grep '\.\./' | grep '^\([A-z]\+\/\)\+[A-z]\+:'
...
test/Makefile:4:        ../railing/railing image -ltest_server -lcompiler -lcrypto -lsyntax_tools -lsasl -los_mon -ipriv -m 1024
```