tests: py: disable main_signon check for printenv cmd

if CONFIG_VERSION_VARIABLE is set, the U-Boot environment
contains a "vers" variable with the current U-Boot version
string. If now "printenv" is called, test/py fails as it
detects the main_sign string, which is in this case correct.

So check only the main_sign as an error, if CONFIG_VERSION_VARIABLE
is not set.

Signed-off-by: Heiko Schocher <hs@denx.de>