I'm agree with this feature but there are some differences from pkgutils and pkgmk.conf usage for MAKEFLAGS.
Since pkgmk is a shell script written in bash it requires a definition of MAKEFLAGS for the existing environment executed in bash, in other words you need to export MAKEFLAGS.
In our case, we are not in a shell script enviroment so we can use directly MAKEFLAGS.

For example, if I'm right, theorically you can compile initramfs.git with 'make MAKEFLAGS="-j2"' and that be used every time you call make.
I tried it and worked but not fully. This is due to some parts must require to be forced to '-j1', like 'make install' when build 'dialog'.
Also 'busybox' failed to compile if '-j1' is not forced to use.

So I think we can start to implement this feature but IMHO we don't need to 'replicate' the MAKEFLAGS variable for our source tree, we are not in a shell script environment as I said first, just we need to force the parts that require '-j1' and let users to export MAKEFLAGS as their needs, but that doesn't means we cannot add some documentation.

also I suggest to change the 'task category' for this ticket and also create a list of tickets related to this one where we should review the build process when using MAKEFLAGS

what do you think about to take this task as general involving all our source tree? in this case we should change 'task category', summary and description to fit the needs

as a side note I started with initramfs/initrd and worked fine, next step should be kernels and look for more candidates

I changed all as I said

it would be fine to note all changes we made

initramfs:
https://crux-arm.nu/gitweb?p=initramfs.git;a=commitdiff;h=3e88256fee44ecb48cecd8c2cb78cc53f71205fb

initrd:
https://crux-arm.nu/gitweb?p=initrd.git;a=commitdiff;h=07bbf582863b81ea9032177bb39662c58d646427