arch: arm: fsl-layerscape: allow "opt-out" of fsl_setenv_bootcmd

arch: arm: fsl-layerscape: allow "opt-out" of fsl_setenv_bootcmd

Allow individual Layerscape boards to opt-out of fsl_setenv_bootcmd
by declaring the original function as weak.

fsl_setenv_bootcmd is used to change the bootcmd based on the
TF-A boot source (e.g QSPI vs SD/MMC) for reasons including
secure boot / integrity measurements and DPAA2 configuration loading.
See previous discussion at [1].

On the Ten64 board, our bootcmd is the same across
all TF-A boot sources so we don't want this behaviour.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
[1] https://patchwork.ozlabs.org/project/uboot/patch/20211110044639.7070-3-matt@traverse.com.au/#2790037

Reviewed-by: Peng Fan <peng.fan@nxp.com>