arm: xea: Modify board code to generate single binary u-boot

This change provides the possibility to build XEA (imx287 based) board
U-Boot as a single binary (without support for CONFIG_SPL_FRAMEWORK).

The generated u-boot.sb can be used in the factory environment to for
example perform initial setup or HW testing.

It can be used with 'uuu' utility
(SDPS: boot -f /srv/tftp/xea/u-boot.sb)

In the configs/imx28_xea_defconfig one needs to disable following configs:
# CONFIG_SPL_BLK is not set
# CONFIG_SPL_FRAMEWORK is not set

The board_init_ll() is used in arch/arm/cpu/arm926ejs/mxs/start.S, which
is utilized when CONFIG_SPL_FRAMEWORK is disabled.

However, when it is enabled - the arch/arm/cpu/arm926ejs/start.S is used,
which requires the lowlevel_init() function.

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