u-boot bbappend causes file system corruption

Question

I need to configure u-boot to boot immediately (disabling the press key to interrupt prompt) for a yocto build.

I've added the following bbappend to do so:

# u-boot_%.bbappend

FILESEXTRAPATHS_prepend := "${THISDIR}/u-boot:"

SRC_URI_append_edge = " \
  file://disable-boot-delay.cfg \
"

where disable-boot-delay.cfg looks like this:

CONFIG_AUTOBOOT_KEYED=y
CONFIG_AUTOBOOT_KEYED_CTRLC=n
CONFIG_BOOTDELAY=-2

This successfully disables the boot prompt, but also causes the FAT partition that u-boot is installed on to become corrupt:

$ # without above bbappend
$ dosfsck /dev/mapper/loop0p1
fsck.fat 4.1 (2017-01-24)
/dev/mapper/loop0p1: 49 files, 6510/20431 clusters

$ # with above bbappend
$ dosfsck /dev/mapper/loop0p1
fsck.fat 4.1 (2017-01-24)
/OVERLAYS/.
  Bad short file name (.).
1) Drop file
2) Rename file
3) Auto-rename
4) Keep it
? 4
/OVERLAYS/..
  Bad short file name (..).
1) Drop file
2) Rename file
3) Auto-rename
4) Keep it
? 4
/dev/mapper/loop0p1: 49 files, 6510/20431 clusters

I'm massively, massively confused as to how this change could be causing file system corruption.

Environment:

• Yocto Warrior (2.7)
• meta-raspberrypi (warrior branch)
• RPI_USE_U_BOOT = "1"

Diffing workdir/build/.config with and without the above append:

229c229
< CONFIG_BOOTDELAY=2
---
> CONFIG_BOOTDELAY=-2
306c306,311
< # CONFIG_AUTOBOOT_KEYED is not set
---
> CONFIG_AUTOBOOT_KEYED=y
> CONFIG_AUTOBOOT_PROMPT="Autoboot in %d seconds\n"
> # CONFIG_AUTOBOOT_ENCRYPTION is not set
> CONFIG_AUTOBOOT_DELAY_STR=""
> CONFIG_AUTOBOOT_STOP_STR=""
> # CONFIG_AUTOBOOT_KEYED_CTRLC is not set