board: Fix documentation for Snapdragon based Samsung and Qualcomm boards

The current documentation for Snapdragon based Samsung
and Qualcomm boards is vague in the sense that at one place
it mentions that u-boot  can be used as a replacement for ABL
bootloader and at another it mentions that u-boot is loaded
as an Android boot image through ABL.

Fix the same.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@linaro.org>