export-image: generate bmap file if bmaptool is available

Higher-capacity cards (particularly Raspberry Pi 128GB A2) have an 8MB
preferred erase size, which is a good indication of the flash block size.

Other large cards may also benefit.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

README.md

@@ -22,7 +22,7 @@ To install the required dependencies for `pi-gen` you should run:
 ```bash
 apt-get install coreutils quilt parted qemu-user-static debootstrap zerofree zip \
 dosfstools libarchive-tools libcap2-bin grep rsync xz-utils file git curl bc \
-gpg pigz xxd arch-test
+gpg pigz xxd arch-test bmap-tools
 ```
 
 The file `depends` contains a list of tools needed.  The format of this

export-image/05-finalise/01-run.sh

@@ -3,6 +3,7 @@
 IMG_FILE="${STAGE_WORK_DIR}/${IMG_FILENAME}${IMG_SUFFIX}.img"
 INFO_FILE="${STAGE_WORK_DIR}/${IMG_FILENAME}${IMG_SUFFIX}.info"
 SBOM_FILE="${STAGE_WORK_DIR}/${IMG_FILENAME}${IMG_SUFFIX}.sbom"
+BMAP_FILE="${STAGE_WORK_DIR}/${IMG_FILENAME}${IMG_SUFFIX}.bmap"
 
 on_chroot << EOF
 update-initramfs -k all -c
@@ -100,6 +101,12 @@ zerofree "${ROOT_DEV}"
 
 unmount_image "${IMG_FILE}"
 
+if hash bmaptool 2>/dev/null; then
+	bmaptool create \
+		-o "${BMAP_FILE}" \
+		"${IMG_FILE}"
+fi
+
 mkdir -p "${DEPLOY_DIR}"
 
 rm -f "${DEPLOY_DIR}/${ARCHIVE_FILENAME}${IMG_SUFFIX}.*"
@@ -128,4 +135,7 @@ esac
 if [ -f "${SBOM_FILE}" ]; then
 	xz -c "${SBOM_FILE}" > "$DEPLOY_DIR/image_$(basename "${SBOM_FILE}").xz"
 fi
+if [ -f "${BMAP_FILE}" ]; then
+	xz -c "${BMAP_FILE}" > "$DEPLOY_DIR/image_$(basename "${BMAP_FILE}").xz"
+fi
 cp "$INFO_FILE" "$DEPLOY_DIR/"

export-image/prerun.sh

@@ -13,7 +13,7 @@ BOOT_SIZE="$((512 * 1024 * 1024))"
 ROOT_SIZE=$(du -x --apparent-size -s "${EXPORT_ROOTFS_DIR}" --exclude var/cache/apt/archives --exclude boot/firmware --block-size=1 | cut -f 1)
 
 # All partition sizes and starts will be aligned to this size
-ALIGN="$((4 * 1024 * 1024))"
+ALIGN="$((8 * 1024 * 1024))"
 # Add this much space to the calculated file size. This allows for
 # some overhead (since actual space usage is usually rounded up to the
 # filesystem block size) and gives some free space on the resulting