sstate: Convert to use ':' as a filename sperator and use SSTATE_SWSPEC globally

sstate: Convert to use ':' as a filename sperator and use SSTATE_SWSPEC globally

Currently the code has problems differentiating between "gcc-cross" and "gcc-cross-initial"
sstate files. We could add in a ton of special casing but tests show this isn't scaling
well. Using a more unique separator resolves the issue.

The choice of which separator to use is a hard one. We need something which isn't commonly
used in PN, PV, PR, *_OS and *_ARCH which rules out '-', '_' and it needs to work ok with
webservers/http which makes ';' and '%' harder.

The change also sets SSTATE_SWSPEC globally since writing out differently named siginfo
files for the fetch/unpack/patch tasks is a waste of diskspace, the hashes match for
all PN in the majority of cases and if they don't, its not a big issue as the hash is
different. This makes the results from sstate debugging more understandable.

(From OE-Core rev: 6f823a23c5f1d0ffa0a27db1c1bc1907de788505)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>