There is no need to have BOOT_PARAMS_SIZE known outside of atags.c

atags.c was the only user of KEXEC_BOOT_PARAMS_SIZE and kexec.h
was only included to get that definition.

Signed-off-by: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Acked-by: Uli Luckas <u.luckas@road.de>