gemini: select CONFIG_BLK_DEV_LOOP

gemini: select CONFIG_BLK_DEV_LOOP

The gemini is using split squashfs/jffs2 root filesystems on
all devices, so without CONFIG_BLK_DEV_LOOP the device does
not gain a writeable root filesystem with these boot messages:

mount_root: unable to create loop device
mount_root: jffs2 not ready yet, using temporary tmpfs overlay

and then it never gets out of that. Fix this so we get writeable
rootfs again.

Link: https://github.com/openwrt/openwrt/pull/21748
(cherry picked from commit b8dc7ac9c68d7e3e3a838c2ca32545c57642653b)
Link: https://github.com/openwrt/openwrt/pull/21973
Signed-off-by: Linus Walleij <linusw@kernel.org>