tools/bootconfig: Cleanup bootconfig footer size calculations

There are many same pattern of 8 + BOOTCONFIG_MAGIC_LEN for calculating
the size of bootconfig footer. Use BOOTCONFIG_FOOTER_SIZE macro to
clean up those magic numbers.

Link: https://lore.kernel.org/all/175211425693.2591046.16029516706923643510.stgit@mhiramat.tok.corp.google.com/



Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>