⚠️ [linker script] simplify memory configuration

[stnolting]

This PR changes the default linker script in order to provide a more flexible memory/section/region configuration.

⚠️ The linker script (sw/common/neorv32.ld) now uses variables to define the section sizes and base addresses:

/* Default rom/ram (IMEM/DMEM) sizes */
__neorv32_rom_size = DEFINED(__neorv32_rom_size) ? __neorv32_rom_size : 2048M;
__neorv32_ram_size = DEFINED(__neorv32_ram_size) ? __neorv32_ram_size : 8K;

/* Default HEAP size (= 0; no heap at all) */
__neorv32_heap_size = DEFINED(__neorv32_heap_size) ? __neorv32_heap_size : 0;

/* Default section base addresses - do not change this unless the hardware-defined address space layout is changed! */
__neorv32_rom_base = DEFINED(__neorv32_rom_base) ? __neorv32_rom_base : 0x00000000; /* = VHDL package's "ispace_base_c" */
__neorv32_ram_base = DEFINED(__neorv32_ram_base) ? __neorv32_ram_base : 0x80000000; /* = VHDL package's "dspace_base_c" */

The actual linker script's MEMORY section must not (!!!) be altered any longer.

✨ By having variables for the sizes and base addresses, the setup-specific configuration can be made much simpler by just passing arguments to the application makefile.

Example to set the RAM section (DMEM) to 16kB during invocation of make:

$ make USER_FLAGS+="-Wl,--defsym,__neorv32_rom_size=16k" clean_all exe

This allows to keep to code base clean (no need to change the linker script). All project-related configurations (ISA extensions, optimization, tuning flags, ...) including the memory layout can now be configured by a single project-local makefile of shell script.