tests: py: dfu: Add functionality to set different u-boot's dfu env variable

tests: py: dfu: Add functionality to set different u-boot's dfu env variable

By default (on almost all systems) the dfu env variable, which defines
available alt settings, is named as "dfu_alt_info".

However on some platforms (i.e. Odroid XU3), the 'dfu_alt_info' is concatenated
from other variables - namely 'dfu_alt_boot' and 'dfu_alt_system' at run time
(when one types 'dfu 0 mmc 0' for first time).

'dfu_alt_boot' describes alt settings which depend on boot medium - for example
boot loader's LBA sectors which are different on eMMC and SD card because of e.g.
MBR/GPT.

'dfu_alt_system' describes board agnostic alt settings - like rootfs, kernel.
On such system we can only append/modify this env variable.

Because of the above, we must have way to modify other than "dfu_ale_info"
variable to perform tests.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
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Changes for v3:
- None

Changes for v2:
- Rewrite of "alt_info_env_name" variable description
- Use of get() method on python's dictionary to easily obtain default
  value