We are using sabresd iMx6Q board.The boot loader splash screen is showing for a few seconds and before the kernel splash screen loads, the lcd screen is black for 18 sec.Kindly help us to know why such a delay between the two screens.Any help would be appreciated.

You could try installing bootchart, see here for info. It may also be available from within your Distro. It gives you a graphic representation of what the system is doing during the boot process.

My Mint 17 distro has quite a long period of "inactivity" before it gets to the login screen but it's actually running round and configuring various bits of hardware while looking like it's dead/hung. (Black screen, white cursor flashing top left. The disk light eventually flickers and we're away!)

If I was smart enough I could probably go and comment out bits of start-up script to prevent it having to time-out while trying to configure some nonexistent device which is what I reckon is causing the perceived problem. It was a generic install and I haven't really "tweaked" anything much as yet.

Note that with bootchart installed it'll generate a fresh .png graphic file on each boot. (I think that's the format )
It leaves it in /var/bootchart if I remember, so you need to periodically clear them out if you're booting a lot.

Hope that helps.

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The log is showing "the kernel started" but the lcd screen is blank for 18 sec.

Do you have newer EFI based system?