rockchip: theobroma-systems: fix modified boot_targets detection

rockchip: theobroma-systems: fix modified boot_targets detection

U-Boot proper automatically modifies boot_targets to swap the order in
which MMC storage media are used for standard boot based on which MMC
storage medium was used to load U-Boot proper. This is however only done
if the user has not changed it manually, therefore a check between the
default and current value is done.

This used to work fine until the migration to standard boot where
boot_targets value size in the default environment went above the 32
characters that env_get_default function can return, thus resulting in a
truncated variable.

Therefore the check between default and current value would always fail.

By using the newly added env_get_default_into function, a buffer of
the appropriate size can be allocated on the stack to get the whole
value of boot_targets in the default environment and thus fixing the
check.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>