fng-install.sh: Fix AB installation for imx6 based devices

This is a fix for issue [746-001621] install-script: B Installation does notwork correctly for imx6.

On iMX8 boards, fng-install.sh works well in AB configuration because of U-Boot boot loader, which uses boot*.scr boot script files.

To be precise, current logic for fng-install.sh during AB installation is:

• [iMX8] if boot.scr exists on /boot partition, fng-install.sh renames it to bootB.scr for installation on partition B;
• [iMX6 without U-Boot] if boot.scr cannot be found, fng-install.sh checks whether boot.cfg exists on /boot partition, and in case of success, renames it to bootB.cfg for installation on partition B.

But after U-Boot was ported to iMX6 boards, boot.scr appeared on boot/ partition. As a result, fng-install script started to rename boot.scr for installation on partition B to bootB.scr, but FNGBoot on iMX6 expects bootB.cfg file to be present and fails.

We decided just to make fng-install.sh to additionally rename boot.cfg for installation on partition B to bootB.cfg for iMX6 machines. So no adjustment o FNGBoot is needed.