Initial commit. Unusable Marlin 2.0.5.3 core without any custimization.

Marlin/src/sd/usb_flashdrive/lib-uhs3/README.txt

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+==== USB HOST SHIELD 3.0 LIBRARY ====
+
+The lib-uhs3/ folder contains a subset of the files from the USB Host Shield
+3.0 library:
+
+  https://github.com/felis/UHS30
+
+
+==== LICENSE SUMMARY ====
+
+Source Path:              Repository:                    License:
+------------              -----------                    --------
+usb_flashdrive/lib        github.com/felis/UHS30         GPLv2 or later
+
+
+==== MARLIN INTEGRATION WORK ====
+
+All additional work done to integrate USB into Marlin was performed by
+AlephObjects, Inc. and is licensed under the GPLv3.
+
+This version of UHS3 has been modified for better compatibility with Marlin.
+The upstream version of UHS 3.0 runs a frame timer interrupt every 1 ms to
+handle device polling. This timer interrupt interferes with Marlin's stepper
+IRQ, so the flag USB_HOST_MANUAL_POLL has been added to move the polling to
+the idle task. Additional logic was added to disable and enable the frame
+IRQ.
+
+SKIP_PAGE3F and USB_NO_TEST_UNIT_READY were added to work around bugs with
+certain devices.
+
+-- marcio@alephobjects.com