Simplify ReaR boot with GRUB2 to allow homogenized boot on multiple platforms.

[didacog]

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Issue details:

• Issue type (Bug / Question / Feature Request / Others?): Feature Request
• Impact (Low / High / Critical / Urgent): High
• Brief description of the issue:
  There are different boot methods in ReaR specific for each supported platform, the scope is to simplify those methods using GRUB2 for all of them. In DRLM we are already using it for simplified netboot in multiple platforms, so it can be the same for ISO and USB boot.

Already tested successfully for i386-pc, x86_64-efi and ppc64le architectures using same procedure for ISO booting.

We were already working on it for kind of a DRLM boot method, but from the discussion in: rear/rear#2944 we'll try to make it rear "standard method" to avoid deprecating support for any ReaR supported platform.

Opening this issue in DRLM as I cannot assign any issue myself in rear project.

[didacog]

Tests booting from ISO:

• i386-pc ------------> Ok
• x86_64-efi -------> Ok
• ppc64le -----------> Ok

Tests booting from USB:

• i386-pc ------------> Ok
• x86_64-efi -------> Ok
• ppc64le -----------> Not working... needs more time testing to find the proper options.

No neet to test PXE as we've using it with success for years.