test: bootm: bootm_process_cmdline_env takes flags

test: bootm: bootm_process_cmdline_env takes flags

The function bootm_process_cmdline_env takes flags as its third
parameter, not a bool. Correct the usage by replacing 'true'
with BOOTM_CL_ALL so that the intent is clear.
A similar change was made throughtout this file in the previous
commit to the one mentioned below as being fixed.

Fixes: 4448fe8e4e7c ("bootm: Allow updating the bootargs in a buffer")
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>