Draft: external-hdf: find XSA files in a folder

Previously each variant would be defined in PL_VARIANTS and the XSA files placed into a folder local to the recipe.

With this change, only the folder location PL_VARIANT_DIR is specified. In the simplest case this is the recipe folder, e.g.:

PL_VARIANTS_DIR := "{THISDIR}/{MACHINE}"

It adds Python code that runs before the script functions are called and recursively globs for all XSA files in it. The SRC_URI and FILES variables are updated accordingly. Changes to the XSA files should be noticed by bitbake so it rebuilds everything.

Alternatively one can still specify a single XSA file with HDF_PATH.

The way XSA files are installed into the sysroot should not have changed. The pl-variants file still gets populated.

Code was partly taken from Xilinx' fpga-manager-util_1.0.bb recipe which does a very similar thing.