bootloader: disable BLS for Fedora

As noted inline, the switch to "boot loader spec" grub entries breaks
our setting of the root device.  This happened some time ago, and it's
not 100% clear to me why our existing Fedora builds haven't broken on
this.  However, the new containerfile based builds do seem to be
hitting this.

Disable it for now.

Change-Id: Ia3472947799bb35ffccfa92937cdd0d68b12a25c

diskimage_builder/elements/bootloader/finalise.d/50-bootloader

@@ -201,6 +201,31 @@ function install_grub2 {
     echo 'GRUB_TERMINAL="serial console"' >>/etc/default/grub
     echo 'GRUB_GFXPAYLOAD_LINUX=auto' >>/etc/default/grub
 
+    # NOTE(ianw) : 2021-05-11
+    # On Fedora there has been a switch to BLS
+    #  https://fedoraproject.org/wiki/Changes/BootLoaderSpecByDefault
+    # which means that kernel-install will have created entries in
+    #  /boot/loader/entries/...
+    # and, as the wiki page says
+    #
+    #   The main issue appears to be that a number of places document
+    #   a way to pass kernel options that is no longer working with
+    #   BLS, i.e. edit /etc/default/grub before running
+    #   grub2-mkconfig.
+    #
+    # i.e. exactly what we're doing above to override the root device
+    # in particular.  Override grub2 to use it's old config format and
+    # remove any BLS entries the kernel install made.
+    #
+    # I imagine at some point we will reworking things in a more
+    # general sense to support BLS if that's what everything is
+    # switching to.
+    #
+    if [[ ${DISTRO_NAME} = fedora ]]; then
+        echo 'GRUB_ENABLE_BLSCFG=false' >> /etc/default/grub
+        rm -rf /boot/loader/
+    fi
+
     if [[ -n "${DIB_BOOTLOADER_SERIAL_CONSOLE}" ]]; then
         SERIAL_CONSOLE="${DIB_BOOTLOADER_SERIAL_CONSOLE}"
     elif [[ "powerpc ppc64 ppc64le" =~ "$ARCH" ]]; then