Shell commands and modules added during link time

[kasjer]

Shell commands registration

So far shell commands could be added with shell_cmd_register() and modules (set of commands) could be added with shell_register()

This approach requires function call usually added in package initialization function.
Registering commands requires some RAM two tables:

static struct shell_module shell_modules[MYNEWT_VAL(SHELL_MAX_MODULES)];
static struct shell_cmd compat_commands[MYNEWT_VAL(SHELL_MAX_COMPAT_COMMANDS) + 1];

Number of modules that can be registered is defined by two mynewt values SHELL_MAX_MODULES and SHELL_MAX_COMPAT_COMMANDS.
When code tries to add more commands or modules system will crash with assert(0).

Link time table for commands registration

Now when link time table can be used shell commands and modules can be added to be available in shell.
Commands can be added with macro MAKE_SHELL_CMD() and modules can be added using SHELL_MODULE_WITH_TABLE().
Both register commands and modules does not use RAM and do not need function calls.

For backward compatibility shell_cmd_register() and shell_register() can still be used.
To minimize memory usage syscfg values SHELL_MAX_MODULES and SHELL_MAX_COMPAT_COMMANDS can be set to 0.

Benefits of new shell commands registration

• Memory usage is reduced both RAM and flash
• commands are in alphabetical order (they were displayed in order they were registered)
• Targets don't need to specify maximum number of command and modules so adding some package will not result in unexpected assert(0) that instruct you to increase limits.

Scope of this PR

• Macros for module and commands creation
• Rework of shell command handling to use link time tables
• Update of many packages (not all yet) to use new way of making shell commands

Comparison of size for target that with many commands

Code without this changes

objsize
   text	   data	    bss	    dec	    hex	filename
 121368	   1744	  30572	 153684	  25854	bin/targets/blackpill411ce-slinky/app/@apache-mynewt-core/apps/slinky/slinky.elf

Same application after change are applied:

objsize
   text	   data	    bss	    dec	    hex	filename
 120844	   1496	  29988	 152328	  25308	bin/targets/blackpill411ce-slinky/app/@apache-mynewt-core/apps/slinky/slinky.elf

[kasjer]

@rymanluk, PR is stripped down now to shell changes only and two examples that shows how os commands tasks, mpool, reset... are converted and module prompt.
Changes to many other commands that are in mynewt-core will follow once basics are accepted.