WIP: Cross arch buildiso support with "tftpboot-installation" to "mkloaders" script

[meaksh]

This is a spinoff PR from #67 , in order to separate this possible approach the cross arch buildiso problems out of the context of the other PR.

The problem: the "grub2-xxxx" packages for all different architectures are not available in the OS repositories for a particular architecture. So, "grub2-powerpc-ieee1275" is not available on the "x86_64" repositories. These packages provides necessary boot/grub files to allow "cobbler mkloaders" to prepare necessary bootloaders for the different architectures.

Current approeaches to solve this problem:

• Create script to setup necessary environment based on "tftpboot-installation" packages that are available on each architecture for all different archs.
• Add the missing "grub2-xxxxx" packages to SUSE/Uyuni Server channels, so we have them available in the SRV. This is the approach that we are currently following in the SUMA Proxy, where these packages were added, due to Retail product requirements.

I'm creating this Draft PR here so we can keep the WIP script until we take a decision.