Regression: uefi tftp+http network boot broken in master (bisected)

[asavah]

printf changes in grub git seem to have broken uefi network boot.

last good commit 968de8c23c1cba0f18230f778ebcf6c412ec8ec5
first bad commit 83603bea6ce8fdff5ab3fbc4c9e592a8c71a8706

I did not test commits in between as they seem to be parts of the same 
thing.
During testing the machine was rebooted many times, including cold boots 
to try to avoid possible firmware glitches.

How to reproduce:
Build grub from git, --with-platform=efi.
Build grub image grub-mkimage -c "embed.cfg" -p "/netboot/current" -d 
"/test_prefix/lib/grub/x86_64-efi" --format=x86_64-efi 
--output=grubx64-test.efi $(ls /test_prefix/lib/grub/x86_64-efi| sed -n 
's/\.mod//gp')
The contents of embed.cfg are as follows:
set root=(http,192.168.0.5)
set prefix=/netboot/current

Have a grub.cfg (doesn't matter whats there) and kernel+initrd on the 
http server.

Try booting the machine via uefi pxe.

Expected result:
The machine boots, grabs grub.cfg and whatnot from the http server and 
presents the menu, works up to and including 
968de8c23c1cba0f18230f778ebcf6c412ec8ec5 .

Actual result:
The machine waits for a while with a black screen and a after that falls 
back to grub prompt.
No http requests can be seen in http server log or tcpdump.
Adding set debug=all to embed.cfg produces:
kern/disc.c:196: Opening `(http,192.168.36.5)' ...
kern/disc.c:281: Opening `(http,192.168.36.5)' failed.
kern/disc.c:295: Closing `http'.
net/net.c:1512: error receiving: 28: couldn't send network packet
* repeats last line many times