thor:cmd: get the default command arguments from environment

thor:cmd: get the default command arguments from environment

This change adds support to getting the default DFU cmd line
arguments from the environment.

DFU and THOR uses the same command line arguments,
so the DFU command environment setup can be used also with THOR.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Conflicts:
	common/cmd_thordown.c

[jino.cho: Fixed conflicts from original commit of tizen.org]
Signed-off-by: jino.cho <jino.cho@samsung.com>
Change-Id: I167927ff47f7c105fd1794da21c6742a10c7593a