bitbake: data: Add support for new BB_HASH_CODEPARSER_VALS for cache optimisation

Currently the codeparser cache ends up being extended for every parse run
since there are values in the functions such as the result of os.getpid()
from LOGFIFO in OE-Core.

Digging into that issue, there are also lots of similar but different
functions being parsed where the change might just be a path to WORKDIR,
a change in PN or PV or something like DATE/TIME.

There is no reason we have to use these changing values when computing the
dependenies of the functions. Even with a small tweak like:

BB_HASH_CODEPARSER_VALS = "LOGFIFO=/ T=/ WORKDIR=/ DATE=1234 TIME=1234 PV=0.0-1 PN=nopn"

the cache is reduced from ~4.6MB, increasing by ~300kb for every parse run
to around 1.3MB and remaining static for oe-core and meta-oe. In my local
build, admittedly heavily experimented with, the cache had grown to 120MB.

The benefits of doing this are:

 * faster load time for bitbake since the cache is smaller to read from disk
   and load into memory
 * being able to skip saving the cache upon shutdown
 * lower memory footprint for bitbake
 * faster codeparser data lookups (since there is less data to search)

We only use these special values when passing code fragments to the codeparser
to parse so the real variable values should otherwise be used in the hash data.

The overall effect of this change, combined with others to avoid saving unchanged
cache files can be ~2s on a ~16s parse on my local system and results in a more
responsive feeling bitbake. It also allows parsing performance to be investigated
more consistently.

(Bitbake rev: f24bbaaddb36f479a59a958e7fc90ef454c19473)

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

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