ACPI: improve acpi_extract_package() utility

The current version requires one to know the size of the package
a priori; this is almost impossible if the package is composed of
strings of variable length.  This change allows the utility to
allocate a buffer of the proper size if asked.

Signed-off-by: Al Stone <al.stone@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>