refactor(memmap)!: change behavioural flags to commands

This change factors out the following memory map tool flags into
independent commands:

- `--footprint` (becomes `memory footprint`)
- `--tree` (becomes `memory tree`)
- `--symbol` (becomes `memory symbol`)

So, for example, where previously you would generate the memory
footprint of a build with:

    memory --tree

You would now instead use:

    memory footprint

Any flags specific to a command (e.g. `--depth` for `tree`) must be
specified after the command, e.g.

    memory tree --depth 1

... instead of:

    memory --depth 1 tree

BREAKING-CHANGE: The image memory map visualization tool now uses
  commands, rather than arguments, to determine the behaviour of the
  script. See the commit message for further details.

Change-Id: I11d54d1f6276b8447bdfb8496544ab80399459ac
Signed-off-by: Chris Kay <chris.kay@arm.com>
5 files changed