If you have not yet upgraded your RPi1 there is a solution to keep you active. Place the following information in a file named "/etc/apt/preferences"
Package: raspberrypi-bootloader
Pin: version 1.20150421-1*
Pin-Priority: 1001
This will prevent the kernel upgrade that has killed the u-boot based boot system the RPi1 has been using for nearly 2 years and permit you to still use apt-get upgrade.
Do NOT attempt to create new diskless images for the rpi1, they will not boot the new kernels after announcing to the router.
It turns out the Raspbian based RPi2 images will also boot the RPi1, so the RPi1 can have a new life beyond u-boot. I *highly* recommend upgrading to the RPi2 as the user experience is greatly improved.
J.