prepare-logo.sh: clean boot directory and boot script if logo file was not found

This is the iMX6 part of the fix for issue [746-001643] bootlogo improvements.

Before spending time on integration of custom logo functionality on iMX8, we want to get a confirmation whether the solution is ok.

To make bootlogo in fngboot optional (to avoid failure when logo.dat is not found), we:

• check whether logo.png is present in boot directory;
• if not, we remove a command line argument in charge of boot logo from boot script. Plus we remove logo.dat from boot directory as well.