xea: config: Use CONFIG_PREBOOT from Kconfig

The usage of the preboot feature is now controlled via a separate Kconfig
option - namely CONFIG_USE_PREBOOT.
It must be enabled for preboot code executing commands now defined in
CONFIG_PREBOOT (also moved to the Kconfig).

After defining both CONFIG_USE_PREBOOT and CONFIG_PREBOOT in
imx28_xea_defconfig the define of CONFIG_PREBOOT shall be removed from
xea.h as it is redundant.

Signed-off-by: Lukasz Majewski <lukma@denx.de>