Break circular dependency on u-root

[hugelgupf]

u-root -> dhcp -> u-root is the circular dependency. Which is fine,
except we messed up modules in u-root big time. We need to break the
circular dep for now.

One warning!

uio does not use "uroot.nohwrng" on the cmdline anymore. Now, UROOT_NOHWRNG=$anything has to be specified in the environment (and one can put that in the cmdline as well, actually)

[codecov]

Codecov Report

Merging #429 (7817cfd) into master (95b2ff6) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #429   +/-   ##
=======================================
  Coverage   67.42%   67.42%           
=======================================
  Files          88       88           
  Lines        3665     3665           
=======================================
  Hits         2471     2471           
  Misses       1030     1030           
  Partials      164      164           

Flag	Coverage Δ
integtests	∅ <ø> (∅)
unittests	67.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dhcpv4/bsdp/boot_image.go	73.80% <ø> (ø)
dhcpv4/bsdp/bsdp_option_boot_image_list.go	100.00% <ø> (ø)
dhcpv4/bsdp/bsdp_option_message_type.go	100.00% <ø> (ø)
dhcpv4/bsdp/bsdp_option_misc.go	100.00% <ø> (ø)
dhcpv4/dhcpv4.go	74.49% <ø> (ø)
dhcpv4/nclient4/conn_linux.go	62.22% <ø> (ø)
dhcpv4/nclient4/ipv4.go	95.77% <ø> (ø)
dhcpv4/option_ip.go	86.66% <ø> (ø)
dhcpv4/option_ip_address_lease_time.go	100.00% <ø> (ø)
dhcpv4/option_ips.go	100.00% <ø> (ø)
... and 32 more

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[JohnAZoidberg]

I have no authority to approve this.

[pmazzini]

lgtm