qualcommax: ipq5018: gl-b3000: fix rootfs_size in bootscript

qualcommax: ipq5018: gl-b3000: fix rootfs_size in bootscript

rootfs_size was being set after the bootscript was already copied
to KDIR_TMP/ this was sneaky because it only affected the initial
compile. All subsequent compiles the rootfs_size is correct before
the copy is made and the bug goes un discovered. Hence the reason I
missed it during testing.

this patch fixes the issue and also refactors the
make recipe to update rootfs_size after the copy
is made, and updates the copy exclusively.

Fixes: 3307fe8ee4cf ("qualcommax: ipq50xx: add support for GL.iNET GL-B3000")
Signed-off-by: Scott Mercer <TheRootEd24@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/18556
Signed-off-by: Robert Marko <robimarko@gmail.com>