ARM: imx: cm_fx6: env: don't run boot scripts twice

Boot scripts located in the root directory of the first partition of
USB, mmc, and SATA drives are executed twice: first by the distro boot
command and then by the legacy boot command. This may have weird side
effects if those scripts only change or extend the environment
(including parts of the boot command itself).

Removing the script execution from the legacy boot command has its own
caveats. For instance, the distro boot command may execute the boot.scr
on the mmc drive, then the boot.scr on the SATA drive, before the
legacy boot command actually boots from the mmc drive. However, the
current behavior would only execute the boot.scr once more before the
actual boot, but it does not prevent the script located on the SATA
drive from being executed, and thus, both scripts from being mixed up.

Considering that the legacy boot command is only in place to boot old
(standard) installations, let's go with the resolution having less
custom code and remove the script execution from the legacy boot
command.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>