docs/getting-started: Added FreeBSD sed debug note.

[dondavan]

A bug occurs when I'm following intrutions from getting-started page to build Unikraft through Makefile, when executing make menuconfig:

thhugh@sp-eduroam-192-119 app-helloworld % make menuconfig
make[1]: Entering directory '/Users/thhugh/Desktop/Unikraft/my-unikernel/unikraft'
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
sed: 1: stdin: \1 not defined in the RE
Makefile:516: *** Target architecture () is currently not supported (could not find /Users/thhugh/Desktop/Unikraft/my-unikernel/unikraft/arch///Makefile.uk).. Stop.
make[1]: Leaving directory '/Users/thhugh/Desktop/Unikraft/my-unikernel/unikraft'
make: *** [Makefile:9: menuconfig] Error 2

This happened when unikraft/Makefile use sed to match host device architecture.

My device:
OS X Monterey, Version 12.5.1.

And I got no problem into config menu in my VM, problem was located in different version of sed.
Where OS X default uses OpenBSD sed caused problem, and GNU sed have no issues.
So brew installed GNU sed and export to PATH, replaced OpenBSD sed, bug solved, no problem entering config menu.
Suggested solution tested.
Consider this problem is platfrom specific, so no change made in Makefile but add note at original page.

Signed-off-by: Hugh Xu [email protected]

[StefanJum]

Thank you @dondavan.
Please squash your commits into one, it should follow the structure presented here.
Do you know if this only happend with OS X? I would say something like Some operating systems (e.g. OS X) are shipped with .....

[razvand]

@dondavan , could you please see @StefanJum 's comments and rebase the PR?