[new uImage] Add fit_parse_conf() and fit_parse_subimage() routines

[new uImage] Add fit_parse_conf() and fit_parse_subimage() routines

Introducing routines for parsing new uImage format bootm arguments:
[<addr>]#<conf>		- configuration specification
[<addr>]:<subimg>	- subimage specification

New format images can contain multiple subimages of the same type. For example
a single new format image file can contain three kernels, two ramdisks and a
couple of FDT blobs. Subimage and configuration specifications are extensions
to bootm (and other image-related commands) arguments' syntax that allow to
specify which particular subimage should be operated on.

Subimage specification is used to denote a particular subimage. Configurations
are a bit more complex -- they are used to define a particualr booting setup,
for example a (kernel, fdt blob) pair, or a (kernel, ramdisk, fdt blob) tuple,
etc.

Signed-off-by: Marian Balakowicz <m8@semihalf.com>