environment-setup.d usage in SDKs and eSDK

["Richard Purdie" <richard.purdie@linuxfoundation.org>]

I've just spent far too long staring at this so I'm going to write down
what I found out. I may as well share it publicly.

An eSDK can be used like a normal SDK with an environment file that can
be sourced. The challenge is that an eSDK contains native binaries
rather than the nativesdk variants in the SDK.

This means that where an SDK has nativesdk environment.d files, an eSDK
ends up using "native" environment.d files which are not used in any
other context.

Some recipes like cmake implement both. Some others like icecc or
openssl only have nativesdk variants. This probably needs cleaning up
to be consistent.

It gets worse when you find that buildtools is included in eSDK and its
environment file is sourced. This means you get a mix of nativesdk and
native. If you set OECORE_NATIVE_SYSROOT in buildtools, it corrupts the
eSDK's variable of the same name. This why buildtools has a hardcoded
list of variables it puts in the enrivonment rather than using
environment.d.

It would be better if the two sets of environment files (native and
nativesdk) referenced different variables as currently the two can't
coexist. This also really needs to be fixed.

Cheers,

Richard