siemens: capricorn: protect environment

With ENV_WRITEABLE_LIST only specific environment variables lisetd in
CFG_ENV_FLAGS_LIST_STATIC are read from the u-boot environment storage.
All other environment variables are set to default values and are not
written back to the storage.

The u-boot environment usually stays for the lifetime of the product.
There is no A/B copy mechanism as for the firmware itself. That means
that incompatible changes to environment variables in future u-boot
versions may lead to serious issues if the old environment is used with
a new u-boot version or vice versa.

Having this protection in place ensures that only a limited set of
environment variables are persisted across u-boot versions. All the
macros not listed in CFG_ENV_FLAGS_LIST_STATIC are now part of the
u-boot binary which is redundant and immutable. This guarantees that
the u-boot version and the default values of these environment variables
are always in sync and cannot be changed at runtime.

ustate and rastate are not relevant for u-boot itself. ustate is used
by swupdate which persists the transaction state in the environment.
rastate is a similar variable used by another user space application.

Signed-off-by: Adrian Freihofer <[email protected]>
Signed-off-by: Heiko Schocher <[email protected]>
Reviewed-by: Peng Fan <[email protected]>
Signed-off-by: Adrian Freihofer <[email protected]>
Signed-off-by: Heiko Schocher <[email protected]>
Reviewed-by: Peng Fan <[email protected]>

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