fix(metadata): Support 'repo:some/name' profile names.

This allows using things like 'gentoo:targets/desktop' or
':targets/desktop' in parent files which is much easier to read than the
default format which requires relative or absolute file paths.
clean(profiles): Remove unused x86/arm profiles.

These profiles are not in regular use, drop them to minimize the number
of profiles to rebase onto upstream Gentoo portage profiles.
churn(profiles): Replace releases/10.0 with upstream.

This adds gdbm to the default USE flags which is already enabled on some
packages but not others. Disable for perl and apr-util for consistency.
churn(profiles): Replace features/64bit-native with upstream.
churn(profiles): Replace features/multilib with upstream.
churn(profiles): Replace arch/base with upstream.
add(profiles/coreos): Add new 'coreos' profile tree.

This will replace coreos specific stuff that is scattered around
'default/linux', 'arch/amd64', 'base', and 'targets' so those can be
replaced by the upstream Gentoo versions.
chrun(profiles): Remove cruft from targets/coreos

This removes settings that either do not apply to packages we install or
duplicate settings that are provided elsewhere. No functional changes.
churn(profiles): Move targets/coreos to coreos/targets/generic
churn(profiles): Move default/linux/amd64/10.0/coreos

default/linux/amd64/10.0/coreos and coreos/amd64/generic have swapped.
The old profile remains for compatibility with existing configurations.
churn(profiles): Remove unused profiles from default/linux/amd64/10.0
churn(profiles): Move default/linux/amd64/10.0

Merge the remaining bits of default/linux/amd64/10.0 into the coreos
tree. default/linux/amd64/10.0 is now an alias for coreos/amd64/sdk for
compatibility with existing configs.
Merge pull request #216 from marineam/profiles

Profiles