Support building by release tag

This adds support for building by release tag. To build a specific
release:
1. git checkout esdk-VERSION in the 'sdk' directory.
2. In the buildroot you can now issue
./sdk/build-epiphany-sdk.sh -C -R -c arm-linux-gnueabihf

The -R flag will ensure that all components are checked out to the
release tag.

* build-epiphany-sdk.sh: Add a release (-R) flag. This flag will
propagate (as --release) through the whole build and ensure that
the right release tag, defined in 'define-release.sh', is checked
out for all components.
* build-toolchain.sh: Support --release flag. Pass it on to
get-versions.sh. Also pass 'toolchain-components' to get-versions.sh
* component-versions: Rename to 'toolchain-components'.
* toolchain-components: Add more detailed definitions of
toolchain components.
* sdk-components: New file. Add definitions for epiphany-libs and
parallella-linux.
* define-release.sh: Slight change of version numbering. Add
comments. Export RELEASE_TAG.
* download-toolchain.sh: Support release flag. Replace hardcoded
component downloads with one call to new download_components
function.
(github_tool): Add (optional) release prefix paramater. Adjust for
tag release prefix parameter (if any) when "--release" is set.
We need to do this for repositories with multiple heads (currently
only 'epiphany-binutils-gdb'.
(download_components): New function. It will download all
toolchain components defined in 'toolchain-components'.
* get-versions.sh: Add mandatory componentfile argument. Support
release flag. Adjust release prefix parameter if needed when in
release mode.
* install-sdk.sh: Remove -d flag. Rename --release flag to
--no-debug. Add new --release flag for release mode. Use
get-components.sh to checkout right version SDK components defined
in 'sdk-components'.