install-buildtools: fix "test installation" step

install-buildtools: fix "test installation" step

The "Test installation" step fails with some harmless error messages
(see [1]). This can however make a user think that the buildtools
have not been installed correctly.

Two reasons for the error messages:
- some envvars in the environment-setup-<arch>-pokysdk-linux file
  start and end with double quotes (e.g., PATH) and are as such
  written into python os.environ. This leads that their usage is
  not valid later when testing the installation. This patch removes
  the double quotes before writing, if they are present.
- if installation directory (install_dir), given through the option
  --directory, is given as a relative path, checking if the path to
  a tool (e.g., gcc) in buildtools starts it will always fail. This
  patch converts the install_dir variable to an absolute path.

[1]
ERROR: Something went wrong: tar not found in ./build-tools
ERROR: Something went wrong: installation failed

Signed-off-by: Aleksandar Nikolic <aleksandar.nikolic@zeiss.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>