openjdk: fix do_install for out of source builds

Hi

Between daisy and dizzy the directory of the build output changed from ${S}/build to ${WORKDIR}/build/
Commit d2b75b615e4612f9fa05950c1d76d4a719e573d4 changes the install task to use the new definition making meta-java work on dizzy but no longer on older versions.
The variable B contains the relevant path already so using ${B} for either ${S}/build or ${WORKDIR}/build keeps compatibility with new and old.

Regards
Max