rename powerpc-softfloat rootfs and installer files to u-boot-arch

The rootfs and installer files (including demo OS) for
powerpc-softfloat are generic enough to use on all known U-Boot based
systems, like ARM.

Making these scripts and files common allows both powerpc-softfloat
and armv7a platforms to use the same files.
onie-mk-bin.sh: add new format 'uboot_ubootenv-up'

This image format is a discontinuous image format for platforms that
locate the boot loader at 0x00000000.

This format emits two binaries:

1.  u-boot + u-boot-env in one binary: onie-<machine>.bin.uboot+env
2.  ONIE kernel+initramfs in another binary: onie-<machine>.bin

In increasing physical address space the order of u-boot+env is:

   low : u-boot
   high: u-boot env

This is in contract to the 'uboot_ubootenv' image format, which also
emits two binaries.  In that case the layout is:

   low : u-boot env
   high: u-boot
qemu_armv7a: update INSTALL instructions

Update the INSTALL instructions for the qemu_armv7a ONIE machine,
including:

- building and patching qemu-2.1.2
- creating disk and flash images
- setting the EEPROM contents
- launching VM using qemu