target: msm8974: Modify ctrl sequence of target_backlight_ctrl

Program WLED modulator enable registers using
pm8x41_wled_led_mod_enable API to change brightness
in bootloader.

CRs-Fixed: 584769
Change-Id: I19edc2ebaf1569c6c55aea7c3c8f4f26e7e70796
app: aboot: add target boot params
Make aboot to get target specific boot params so that boot params can be
appended to kernel command line.
Also fix mdmkrypton to provide 'root=' as target boot param to get rid of
hardcoded root device name from boot image.

Change-Id: I10129a0249438b8bc7da1ea8a8c0097489e6e289
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>