release: correct Azure disk image extension

[emaste]

Summary

Azure images are Microsoft VHD files which use the .vhd extension.
We use the mkimg format arg "vhdf" to specify the fixed vhd format, but
the extension should remain .vhd.

[lwhsu]

I'm fine with this. For the record, when I worked on this, I kept azure.vhdf as it is an intermediate product and for aligning with other images. Note that the azure-do-upload target in Makefile.azure will upload as .vhd suffix, with more clear naming.

[cperciva]

I garbage collected the AZURE_DISK variable since it was unused. (Note that CLOUDWARE does not include AZURE so in fact *none* of the cloudware-related AZURE variables are used, but _DISK in particular isn't used for any of the cloudwares.)

The AZURE_IMG bit needs to change since we now have support for building separate UFS and ZFS images (with different file names).

[whu]

Thanks!