Bring linker scripts in sync with pico-sdk original.

[mvds00]

The linker script apparently evolved in pico-sdk. This patch updates the linker script in picowota to match the original script (memmap_default.ld) as closely as possible.

This will make it easier to spot issues as either project evolves.

A related goal is to have some way of using the pico-sdk linker scripts as a template for the linker scripts in e.g. picowota (see templated linker scripts, issue #398 of pico-sdk).