Disable debug output DEBUG_CMD
Delete imgtool.exe when making clean
Fix linker errors (firmware)

With avr-gcc 4.8.2 appeared some trouble:
provider.c:120:(.text+0x282): relocation truncated to fit: R_AVR_7_PCREL
against `no symbol'
collect2: error: ld returned 1 exit status

Rewriting Makefiles to separate compilation and linking fixes this.
Introduces new features:

- automatic dependencies
- settings move to ~/.xd2031/firmware.conf
  (this keeps the source directories clean and enables easy updating
   via git pull):
- make update
- make size (closes #118)
- make relsize
Fix broken firmware compile (AVR: util/delay.h)