mdloader can't find applet-flash-samd51j18a.bin unless in working directory

[tzarc]

At least in the case of Windows, mdloader is incapable of programming a CTRL unless applet-flash-samd51j18a.bin is in the current working directory.

We've been attempting to allow for QMK to use the :flash target to invoke mdloader, but given that it's executed from the qmk_firmware directory, applet-flash-samd51j18a.bin doesn't exist.

However, if executing mdloader from a directory that contains applet-flash-samd51j18a.bin, this works. Having applet-flash-samd51j18a.bin present next to mdloader does not result in a successful flash.

Perhaps searching the executable directory for a copy of applet-flash-samd51j18a.bin might be in order?

[fauxpark]

Looking at how avrdude handles finding its conf file - it seems that on Windows, it uses SearchPath() and thus avrdude.conf must be in the %PATH% or the CWD. On Linux, the location is derived from sysconfdir provided by make: https://www.gnu.org/prep/standards/html_node/Directory-Variables.html

mdloader could possibly do a similar thing to resolve the applet, but use datadir for /usr/local/share on Linux. This would greatly help with packaging for Homebrew, MSYS2 and the various Linux distros.