h8300/boot: Use POSIX "$((..))" instead of bashism "$[...]"

On Ubuntu, where /bin/sh -> dash, "make ARCH=h8300 clean" gives:

printf: 1: $[0x00400000+0x00140000]: expected numeric value

Replace the bash-specific "$[...]" by POSIX "$((..))" for arithmetic
expansion to fix this.
Note that according to the bash 4.1 manpage, "$[...]" is deprecated, and
will be removed in upcoming versions of bash.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>