Cleanups in bootm command and related code

Cleanups in bootm command and related code

Instead of selecting supported header type (standard U-Boot image
or custom TP-Link) at compile time, from now both headers are
supported. This allows to use both image types on all devices,
which sometimes might be useful.

Other changes:

- clean up code style, use tab size = 8
- restore support for Multi-File image, used in some ZBT routers
- add new TP-Link image header definition, get rid of old one
- restore CRC checksum tests (for image header and data)
- make image header information look better, include more data
- remove bootd command, as it was only alias for boot command