boot: change some arch ifdefs to feature ifdefs

The routines boot_ramdisk_high, boot_get_cmdline and boot_get_kbd
are currently enabled by various combinations of CONFIG_M68K,
CONFIG_POWERPC and CONFIG_SPARC.

Use CONFIG_SYS_BOOT_<FEATURE> defines instead.

CONFIG_SYS_BOOT_RAMDISK_HIGH
CONFIG_SYS_BOOT_GET_CMDLINE
CONFIG_SYS_BOOT_GET_KBD

Define these as appropriate in arch/include/asm/config.h files.

Signed-off-by: John Rigby <john.rigby@linaro.org>
Acked-by: Wolfgang Denk <wd@denx.de>