ipxedust: add nap.h to arm64 build to set NAP_NULL to support u-boot

[rpardini]

ipxedust: add nap.h to arm64 build to set NAP_NULL to support u-boot

• see iPXE stucks once it tries to wait for user input when loaded by ARM64 U-Boot's bootefi ipxe/ipxe#605 (reply in thread)
• "This will cause iPXE to leave the CPU spinning instead of putting it to sleep, and will avoid the problems on U-Boot."
• Seems harmless on non U-Boot machines -- the CPU will spin during the key timeout (2s?) but that's about it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.82%. Comparing base (52f7e0f) to head (fc5ed25).
Report is 7 commits behind head on main.

❗ Current head fc5ed25 differs from pull request most recent head 3db501c. Consider uploading reports for the commit 3db501c to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #152   +/-   ##
=======================================
  Coverage   97.82%   97.82%           
=======================================
  Files           5        5           
  Lines         367      367           
=======================================
  Hits          359      359           
  Misses          4        4           
  Partials        4        4           

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