Unified command execution in one place

At present two parsers have similar code to execute commands. Also cmd_usage() is called all over the place. This series adds a single function which processes commands called cmd_process().

This new function understands return codes, and in particular CMD_RET_USAGE to indicate a usage error. So rather than calling cmd_usage() themselves, the command handlers can just return this error.

There appears to be a run_command2() which is used to run commands with the selected parser. This series changes this in two separate steps to just run_command(), and renames the old run_command() to builtin_run_command(). No one should call this outside main.c since if the hush parser is being used it is wrong to call it. The built-in parser code could move into a separate file perhaps in a future patch.

The overall series reduces code size on ARM by about 1KB on my ~160KB U-Boot text region when the hush parser is used, and around 60 bytes when it isn't.

As an aside the only user of parse_line() is fsl_ddr_interactive() which seems to have its own command line interface which operates before DRAM is set up. Do I have this right? Is there no way this could be done later from a normal U-Boot command?