bootscript: merge bootscript and bootscript-alt recipes

This is a fix for issue https://secogroup.atlassian.net/browse/YT-153

In this MR, the bootscript-alt with bootscript are merged into bootsript to reuse the same package both in FNG System and Yocto.
Same was done for bootscript-uboot-alt and bootscript-uboot recipes.

Herewith a boot-alt.cfg was relocated to bootscript recipe as is, as it is not possible to merge it with boot.cfg provided by bootscript.
Same applies for boot-alt.scr.cfg and boot.scr.cfg.