Adding variables that can be set from other linker scripts:
- _bootloader_head_size:
Bootloader flash offset in front of the application.
- _bootloader_tail_size:
Bootloader offset from the tail of the flash.
In case the bootloader is located at the end.
- _bootloader_head_ram_size:
Bootloader RAM usage in front of the application.
Updated calculations of application flash and RAM.
With all the variation in chips and boards it's tedious to copy and
redefine linker scripts for every option. Making linker scripts more
modular also opens up more possibilities, like enabling/disabling the
flash file system from the Makefile - or even defining it's size from a
Makefile argument (FS_SIZE=12 for a 12kB filesystem if tight on space).
