[cloud] Add utility for importing images to Google Compute Engine

[cloud] Add utility for importing images to Google Compute Engine

Following the example of aws-import, add a utility that can be used to
upload an iPXE disk image to Google Compute Engine as a bootable
image.  For example:

  make CONFIG=cloud EMBED=config/cloud/gce.ipxe \
       bin-x86_64-pcbios/ipxe.usb bin-x86_64-efi/ipxe.usb

  make CONFIG=cloud EMBED=config/cloud/gce.ipxe \
       CROSS=aarch64-linux-gnu- bin-arm64-efi/ipxe.usb

  ../contrib/cloud/gce-import -p \
       bin-x86_64-pcbios/ipxe.usb \
       bin-x86_64-efi/ipxe.usb \
       bin-arm64-efi/ipxe.usb

The iPXE disk image is automatically wrapped into a tarball containing
a single file named "disk.raw", uploaded to a temporary bucket in
Google Cloud Storage, and used to create a bootable image.  The
temporary bucket is deleted after use.

An appropriate image family name is identified automatically: "ipxe"
for BIOS images, "ipxe-uefi-x86-64" for x86_64 UEFI images, and
"ipxe-uefi-arm64" for AArch64 UEFI images.  This allows the latest
image within each family to be launched within needing to know the
precise image name.

Google Compute Engine images are globally scoped and are available
(and cached upon first use) in all regions.  The initial placement of
the image may be controlled indirectly by using the "--location"
option to specify the Google Cloud Storage location used for the
temporary upload bucket: the image will then be created in the closest
multi-region to the storage location.

Signed-off-by: Michael Brown <mcb30@ipxe.org>