ath79: tiny: set default BLOCKSIZE to 4 KB

ath79: tiny: set default BLOCKSIZE to 4 KB

The NOR Flash mtd erase block size is 4 KB on ath79 tiny sub-target.
Squashfs-split driver always check and create the jffs2 rootfs_data
partition on the first free block. However, sysupgrade script append
the config backup to the end of the sysupgrade image. If we pad the
image to the 64 KB boundary, the kernel will be unable to find a
valid jffs2 partition and then recreate the rootfs_data partition.
Users may lose their config during upgrades. Fix this issue by setting
BLOCKSIZE to 4 KB so that the sysupgrade image can be aligned to the
4 KB boundary.

Fixes: https://github.com/openwrt/openwrt/issues/20495
Fixes: 05d35403b211 ("ath79-tiny: enable 4k sectors")
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/22497
Signed-off-by: Robert Marko <robimarko@gmail.com>