Split up external-sourcery-toolchain into separate recipes

common-license.bbclass will set LIC_FILES_CHKSUM to a common license file
based on LICENSE, which is appropriate for a case where we have no sources to
refer to.
external-toolchain.bbclass, among other things, handles extraction of files in
the external toolchain sysroot, based on patterns in the FILES variables,
checking alternate locations to better support any arbitrary toolchain, and
has basic mirror handling for checking multiple paths within the sysroots.

Under normal circumstances, I'd want this to use highly granular commits, but
this branch has been extremely long lived (>1yr) and is such an invasive
refactoring that attempting to break it down now would be of limited
usefulness.

Signed-off-by: Christopher Larson <chris_larson@mentor.com>