With the move of configure scripts out of target directories, relative
paths to top_srcdir got broken:
/bin/sh: .../newlib/libgloss/../../mkinstalldirs: No such file or directory
Fix the PRU build by switching to srcroot relative path, as rest of targets do.
Fix the Blackfin build in the same way.
Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>