Rainbow: reconsider versioning for all three routers

While reviewing !966 (merged), I noticed that we need to edit all three Makefiles if we bump rainbow-ng.

You should consider similar solution how it is done for shipping Turris u-boot packages in this repository or rather proposed solution, which I got on IRC #openwrt-devel:

20:23:24    <pepes>    Guys, I am thinking. I would like to have dedicated U-boot package installable within opkg and I could copy the U-boot image from staging_dir and put it to whatever I want. Because right now, I compiling it twice. :-( Is there way, how can I use the same versioning from different package? So, I could know the version of U-boot by opkg. Any ideas?
20:24:05    <jow>    pepes: there's no clean way
20:24:23    <jow>    you could define a shared .mk file that just declares the version, then include that in both places
20:25:38    <jow>    or you could use a construct like  PKG_VERSION:=$(if $(DUMP),x,$(shell sed -ne 's#PKG_VERSION:=##p' $(topdir)/package/boot/u-boot-foo/Makefile))
20:28:04    <pepes>    Thanks jow! I will try it, but it looks like it is going to help. THanks! I will let you know about it.

This means use shared .mk file.