Petitboot v1.8.0 tagged

[Samuel Mendoza-Jonas]

Petitboot v1.8.0 has been tagged, notable changes including:

- Fixes for make dist
	`make dist` should actually work now, allowing us to start using release
	tarballs again.

- Fix for boot file verification
	A hole was found in the gpg validation code hidden by some confusing
	indenting where incorrect signatures could be mistakenly marked as
	valid. There were also small fixups to the boot path and the mkstemp
	invocation.

- BLSCFG support added to the GRUB parser
	BLSCFG is a common format designed to be read by other bootloaders. This
	adds support to the GRUB parser to find and parse BLSCFG snippets if
	configured.

- A number of other updates including;
	fixes for musl support, updates to the syslinux-parser, coverity fixups,
	enabling ppc64le on Travis, scripts for buiding Petitboot in Docker

Speaking of make dist working again, Petitboot releases will now be available
as a tarball on the open-power Petitboot repository, for example:
https://github.com/open-power/petitboot/releases/download/v1.8.0/petitboot-1.8.0.tar.gz
In particular this lets us properly verify releases and avoid some autoconf
magic when building using tools such as Buildroot. Potentially we can also look
at hosting tarballs on ozlabs.org as well.