Rework/update/add Readme.md in the platforms directories.

src/platforms/Readme.md

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+# Platforms and platform support files
+
+This directory contains the implementation of platforms and support file
+used by (multiple) platforms.
+
+## Implementation directories
+
+native : Firmware for original Black Magic Probe<br>
+stlink : Firmware for STLINK-V2 and V21<br>
+swlink :  Firmware for STLINK-V1 and Bluepill<br>
+hydrabus :  Firmware https://hydrabus.com/ <br>
+f4discovery    : Firmware for STM32F407DISCO<br>
+launchpad-icdi :<br>
+tm4c: <br>
+hosted: PC-hosted BMP running as PC application talking to firmware BMPs,
+        STLINK-V2/21/3, FTDI MPSSE probes, CMSIS-DAP and JLINK
+
+
+## Support directories
+
+common: libopencm3 based support for firmware BMPs<br>
+stm32:  STM32 specific libopencm3 based support  for firmware BMPs<br>
+pc: Support for PC-hosted BMPs.<br>

src/platforms/f4discovery/Readme

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-System vs BMP Bootloader
-========================
-For the BMP bootloader, flashing was not reliable. So we use the system
-bootloder unconditional.
-
-Connections:
-====================
-
-PA0: User button to force system bootloader entry with reset
-PA2/PA3 eventual connected to the STLINK/ STM32F103C8
-
-PC2: TDI
-PC4: TMS/SWDIO
-PC5: TCK/SWCLK
-PC6: TDO/TRACESWO
-
-PC1: TRST
-PC8: SRST
-
-PD15/Blue Led: Indicator that system bootloader is entered via BMP
-
-Bootstrapping the F4Discovery on-board ST-Link
-==============================================
-http://embdev.net/articles/STM_Discovery_as_Black_Magic_Probe has some hints
-how to modify the F4Discovery on-board ST-Link.  If you try to do so and hit
-a problem that stands some test like that you load the right firmware to the
-right device via the right BMP probe, explain, report and ask on the
-blackmagic mailing list http://sourceforge.net/mail/?group_id=407419.

src/platforms/f4discovery/Readme.md

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+# Firmware BMP for STM32F407 DISCO boards
+
+Kept for historical reasons to load BMP bootloader to the STM32F103 of the onboard STLINK or external STLINKs. As stlink-tool now allows to load BMP firmware via the original STLINK bootloader is no longer really needed.
+
+## Connections:
+
+PC2: TDI<br>
+PC4: TMS/SWDIO<br>
+PC5: TCK/SWCLK<br>
+PC6: TDO/TRACESWO<br>
+
+PC1: TRST<br>
+PC8: SRST<br>