bootimg: Use the same OS files for each boot method

bootimg: Use the same OS files for each boot method

Fixes [YOCTO #1951]

The do_bootimg code can generate hybrid efi+pcbios images (syslinux and
grub-efi) to boot on platforms with both EFI and legacy BIOS options. The
current implementation copies the kernel, initrd, and rootfs twice,
unnecessarily bloating the image size. This is an especially egregious bug
on -sato images.

Update the classes to use a common install of the kernel, initrd, and rootfs to
the root of the boot media. Grub-efi, syslinux, and isolinux can all reference
this location explicitly with a leading slash.

Tested with an EFI+PCBIOS image in both EFI and PCBIOS boot modes on two
platforms. No ISO image testing was performed.

(From OE-Core rev: 5209016cf4c4c8f649e37dc8857b3fbcfe8dd8c8)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>