The boot script appends values to certain variables such as
$extra_bootargs and $overlays.
If the final instruction of the boot script (dboot command)
fails, these variables contain the new values, plus the original
one. Since the user recovers the prompt, he may do a 'saveenv'
to save the environment, and the modified variables will be
saved, only to be enlarged again on the next boot.
This can lead to repeated strings on such variables.
Save the original value and restore it in case of failure on
the dboot command.
Signed-off-by: Hector Palacios <hector.palacios@digi.com>
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