arm/km: introduce bootcount env variable and clean km_arm

This environment variable is used to set the bootcount address
for the kernel.

last_stage_init is not available for arm platforms. So the
calls to set_km_var and set_bootcount_addr are done in
misc_init_r.

Additionally some unneeded printouts were removed.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Acked-by: Heiko Schocher <hs@denx.de>
cc: Wolfgang Denk <wd@denx.de>
cc: Detlev Zundel <dzu@denx.de>
cc: Prafulla Wadaskar <prafulla@marvell.com>