Uses logic from apps/fsutils/ipcfg to obtain the IP address configuration from a file, if available. This only effects the behavior of netinit if CONFIG_FSUTILS_IPCFG is selected and if an IP configuration file is available on a file system. Otherwise, it reverts to the preceding behavior.
Also adds definitions to apps/include/fsutils/ipcfg.h to formalize a bit encoding of the prototype which was already in use, but not documented.
Also fixes some bad assertions (duplication of PR #420) and makes use of variable names consistent in ipcfg.h comments and in ipcfg_text implementation.
