plat: stm32: merge updates from libmaple.

Commits merged (in timeline, latest first):

commit 048e70fc4163d8377b590eb134e4a60f7c48f9c3
Author: Marti Bolivar <mbolivar@leaflabs.com>

    usb_cdcacm: Make USB lib globals weak symbols.

commit 0dbf94bf31077b77ffd893d8396bdfb34ccca341
Author: Marti Bolivar <mbolivar@leaflabs.com>

    usb_cdcacm.c: Fixups for usb_cdcacm_tx().

commit d12a5b14e11b092122a106b5dd9fb92b169eeca6
Author: Marti Bolivar <mbolivar@leaflabs.com>

    usb_reg_map.c: Fix incorrect function name call.

commit 9e14f40c20c5c0d2231836dcde6c7f42080f4c58
Author: Marti Bolivar <mbolivar@leaflabs.com>

    usb_reg_map.h: Fix incorrect register bit definitions.

commit cea8158e97f21bde96f3561baade15400fafdfd2
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Check if the last packet sent was the full endpoint size,
    in this case flush the host buffers by sending a 0 byte packet

commit 74460684001d31f09b06e2a3eeede58b72b65e68
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Handle sending 0 byte packets.

commit 699f97fdcbb886096870cce1fb6d475514998930
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Move endpoint definitions to include files (they are now needed
    outside in usb_serial.cpp)

commit b3287225c5223b29d15279dee8d440c60e1fa983
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Rename misleading ptr function

commit e6a138499d09c205e96c0c5f51dc639dfb5f58de
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Fix BKP write enable

commit bbff6fbf78cd08b412915025ce81a189bc5b085b
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Set DCNTR before starting DMA transfer.

commit c95edd391e2518572b53e934ff554eabd9294c46
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Refactor setting a USB EP rx count

commit e0748ce668907afa54c895dc9b4d0b4e1d19fd00
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Rename USB_EP_EP_KIND to USB_EP_EP_KIND_DBL_BUF

commit 0c8ab1f655470309c9246c9a8547daed38ff1bb0
Author: Manuel Odendahl <wesen@ruinwesen.com>

    Add double buffering routines