arm/km: remove duplicate code

This patch removes 2 small portions of board specific code that are
duplicated with common ARM code and thus unnecessary. These 2 portions
of code concern the mach id that is given by u-boot to the Linux kernel
to identify the system's MACH.

The first one was the possibility to change the MACH thanks to the
'mach_id' enviroment variable. This is possible thanks to the 'machid'
env variable (see arch/arm/lib/bootm.c:boot_jump_linux()).

The seconde one is the setting of the bi_arch_number. This is already
done in arch/arm/lib/board.c:board_init_f() since km_arm.h does #define
CONFIG_MACH_TYPE.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
Acked-by: Prafulla Wadaskar <Prafulla@marvell.com>