Setup for link options for a standard SDK executable.

This includes:

C runtime initialization
Linker scripts for default, no_flash, blocked_ram and copy_to_ram binaries
'Binary Information' support
Linker option control

The default linker scripts comprise of a base memmap_*.ld file, and a series of .incl files that are included by it.

The base memmap_*.ld files are located in the pico_platform directory of the target chip. Each file has a tree showing the files that are included by it.

To override any included files, place the overriding files in a directory, and pass that directory to the pico_add_linker_script_override_path CMake function. The usual files to override when making a custom linker script are:

memory_extra.incl, to add extra memory regions
section_extra_*.incl files, to add extra sections to the binary - see the comments in the files for more details
default_*_excludes.incl files, to change what is placed in RAM for flash binary types (eg default, blocked_ram)

You can also set variables in the linker script using the pico_set_linker_script_var CMake function, to change the location and size of default memory regions. The available variables are:

HEAP_LOC, to set the heap location
HEAP_LIMIT, to set the heap limit
RAM_ORIGIN, to set the origin of RAM
RAM_LENGTH, to set the length of RAM
XIP_RAM_ORIGIN, to set the origin of XIP RAM
XIP_RAM_LENGTH, to set the length of XIP RAM
SCRATCH_X_ORIGIN, to set the origin of scratch x
SCRATCH_X_LENGTH, to set the length of scratch x
SCRATCH_Y_ORIGIN, to set the origin of scratch y
SCRATCH_Y_LENGTH, to set the length of scratch y