sdk: Decouple default install path from built in path

Add SDKPATHINSTALL which is used as the default install location of the SDK
instead of SDKPATH. This means the default install path isn't encoded into
every SDK binary, meaning if a date is used there the entire SDK doesn't
have to rebuild. Most distros can switch to only customise SDKPATHINSTALL
meaning more sstate reuse too.

[YOCTO #14100]

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