Documentation: Use file paths to flashing firmware tutorial

In preperation for CB:62424, replace HTTP links pointing to the flashing
firmware tutorial with file paths to the Markdown files.

Change-Id: I6a271a912348cbe002bc9cced9922ed743e1133c
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62452
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Thomas Heijligen <src@posteo.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>

Documentation/mainboard/msi/ms7707/ms7707.md

@@ -74,7 +74,7 @@ seconds. Setting the jumper alone is not enough (the Fintek is VBAT backed).
 Put all back in place and restart the board. It might need 1-2 AC power cycles
 to reinitialize (running at full fan speed - don't panic).
 * External flashing has been tested with RPi2 without main power connected.
-3.3V provided by RPi2. Read more about flashing methods [here](https://doc.coreboot.org/flash_tutorial/index.html).
+3.3V provided by RPi2. Read more about [flashing methods].
 * In case of going back to proprietary BIOS create/save CMOS settings as early
 as possible (do not leave BIOS on first start without saving settings).
 The BIOS might corrupt nvram (not cmos!) and leave the system in a dead state
@@ -110,3 +110,4 @@ needed (internally re-routed already).
 [Winbond 25Q32BV datasheet]: https://www.winbond.com/resource-files/w25q32bv_revi_100413_wo_automotive.pdf
 [Fintek F71808A datasheet]: https://www.alldatasheet.com/datasheet-pdf/pdf/459069/FINTEK/F71808A.html
 [flashlayout]: flashlayout.svg
+[flashing methods]: ../../../flash_tutorial/index.md