Previously, external modules had to be individually added to both
EXTERNAL_MODULE_DIRS and USEMODULE. If those where not in sync, this
resulted in build errors.
With this commit, search folders for external modules are added to
EXTERNAL_MODULE_DIRS instead. So lets say the file system structure is
like this
```
└── /path/to/external/modules
├── mod_a
│ ├── Makefile
│ ├── Makefile.dep
│ ├── Makefile.include
│ ├── foo.c
│ └── include
│ └── external_module.h
└── mod_b
├── Makefile
└── bar.c
```
One now adds `/path/to/external/modules` to EXTERNAL_MODULES and only
with `USEMODULE += mod_a` the corresponding module, dependencies and
include settings are actually used. Hence, it is possible to configure
`EXTERNAL_MODULE_DIRS` from `~/.profile` or `~/.bashrc` once and never
needs to worry about them again.
