We're having speed issues on the autobuilder due to the numbers of files in sstate
directories. We previously split these by the first two characters of the hash.
This change extends this to split by the next two characters as well, creating
more layers of directories.
This should signifiantly speed up eSDK builds on the autobuilder as the current
sstate layout simply isn't scaling there but addresses a general complaint.
gen-lockedsig-cache needed to be updated for the new split level sstate.
Also update tests for new layout.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
