memtest86plus: add new package

memtest86plus: add new package

Add package for memtest86+ and ability to boot directly into it from
existing grub menu. Both legacy and EFI packages are available and each
will provide the needed memtest86+ images plus a postinst and postrm to
handle the grub.cfg menu entries.

The images are tiny, adding at worst 154 kB to the kernel partition.

I do not have x86 box for testing but the memtest86plus-efi package
works as expected on my test machine (ie booting into the image and the
setup and removal of the grub.cfg menu entry).

Build system: x86/64
Build-tested: x86/64-glibc
Run-tested: x86/64-glibc (Intel N150 based box)

Signed-off-by: John Audia <redacted>