Broadcom NIC stays down in iPXE
You're using the tg3 driver, and it is known to have some issues on certain models of HP hardware. You might have luck if you apply the patches mentioned in this mailing-list thread:
http://lists.ipxe.org/pipermail/ipxe-devel/2012-August/001718.html
You might also get it to work by using undionly.kpxe to avoid using an iPXE-native driver.
I also want to mention that it has been reported in the past that the tg3 driver had problems to get linkup when using gigabit, but it worked on a 100Mbit switch. You might like to try this out just to confirm.