src: wordboot.c: Fix formula for BOOTLOADER_TIMEOUT_COMPARE_VALUE
src: wordboot.c: Use 1024 as prescaler for Timer0

This allows for a wider range of useful values for BOOTLOADER_TIMEOUT_MS.
src: wordboot.c: Remove unused header file
src: wordboot.c: Clear __zero_reg__ manually

This resolves an boot loop issue when performing a actual power-on reset as
the register would not be initialized. Alternatively the -nostartfiles and
-nolibc in the Makefile should be removed.
src: Include normal initialization routines

This change makes sure that normal initilization routines will be added to the
binary, so no weird side-effects will occur and no inline assembler to fix it
is needed. While this increases the code size by a couple of bytes, it still
remains under the 512 words limit, so its not a problem for now.