sstate: Exclude SSTATE_EXTRAPATH from checksums

sstate: Exclude SSTATE_EXTRAPATH from checksums

After the change to allow target recipes to depend on native recipes, the
native checksums becomes all the more critical. Add to this that we're now
accounting for pre/postfuncs and we have a cache reuse issue since the
distro LSB string is getting coded in when it shouldn't be.

This excludes that string and allows one set of native sstate to share
checksums with another set from a different host distro. They're separated
into different directories so this is fine for our use cases.

(From OE-Core rev: 1fa7d4331d994b9eeb6f973d1a1f04cb4df92c13)

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