Add skip-data-init feature to disable .data section initialization

[KushalMeghani1644]

This PR adds an optional feature to skip .data initialization during startup.

Introduces:

• the skip-data-init feature that disables the .data copy loop in the reset handler, for boot flows that already initialize RAM (e.g. RP2040 boot2 that copies to RAM and unmaps flash).

• Adds a compile-time guard preventing skip-data-init from being used with zero-init-ram.

• Includes a minimal example at skip-data-init.rs.

• Default behavior is unchanged; .data is still copied when the feature is not enabled.

This PR tries to address issue #609

[jonathanpallant]

Stubbing out the copy loop seems fine but what is the purpose of the example? It has no observable output and no mechanism to run it with the kind of bootloader that it must be used with to avoid UB. Was it LLM generated?