pkg-waf: add additional variables for each install step

pkg-waf: add additional variables for each install step

Some waf packages may want to pass additional variables to waf script
in install step. Add the possibility to do so by defining the
following variables:

  <pkg>_INSTALL_STAGING_OPTS for the install to staging directory
  <pkg>_INSTALL_TARGET_OPTS for the install to target directory

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>