Add skip-data-copy feature for when the .data LMA isn't initialized after boot.

[BenChung]

This implements the skip-data-copy feature, which disables loading the .data section (containing statically allocated variables) in cases when that section is either:

• Already loaded and possible, or
• No longer valid when the startup code runs.

This occurs for example with boot2 using the BOOT_LOADER_RAM_MEMCPY option, which copies the .data section and then de-initializes XIP before relinquishing control leaving the contents of .data at the LMA undefined.

This fixes #609. Tested on a RP2040 in a pico. The test project is https://github.com/BenChung/RP2040_boot2_run_from_ram, which panics without this fix, or works correctly with this patch (shown on the branch https://github.com/BenChung/RP2040_boot2_run_from_ram/tree/fixed).