Flash lab: make sure people don't use U-Boot from MMC

- Will need to investigate the issue
  With U-Boot build from the MMC configuration, U-Boot
  sees a subpage of 512 bytes (instead of 2048) on the Xplained board,
  and this is incompatible with what Linux sees. Hence U-Boot
  cannot use a UBI space that Linux can use.

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

labs/sysdev-flash-filesystems/sysdev-flash-filesystems.tex

@@ -12,6 +12,11 @@ \section{Setup}
 \code{mtd-utils} package, which will be useful to create UBIFS and
 UBI images.
 
+Also remove the SD card, to resume to the former way of booting.
+That's important because there is currently an issue with U-Boot
+compiled from the MMC configuration, which causes UBI incompatibilities
+between U-Boot and Linux.
+
 \section{Goals}
 
 Instead of using an external SD card as in the previous lab, we will