pkg-generic: add step_pkg_size global instrumentation hook

This patch adds a global instrumentation hook that collects the list
of files installed in $(TARGET_DIR) by each package, and stores this
list into a file called $(BUILD_DIR)/packages-file-list.txt. It can
later be used to determine the size contribution of each package to
the target root filesystem.

Note that in order to detect if a file installed by one package is
later overriden by another package, we calculate the md5 of installed
files and compare them at each installation of a new package.

Collecting the list of files installed by each package is done
unconditionally, as tests have shown that the performance impact of
doing this is negligible.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Ryan Barnett <ryanbarnett3@gmail.com>
Tested-by: Ryan Barnett <ryanbarnett3@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>