This is discouraged by buildroot manual page.
https://buildroot.org/downloads/manual/manual.html says:
'You should never use make -jN with Buildroot: top-level parallel make is currently not supported.
Instead, use the BR2_JLEVEL option to tell Buildroot to run the compilation of each individual
package with make -jN'
There is already a config parameter in buildroot ( BR2_JLEVEL ) that is by default set to 0. Help for this option says 'If 0, determine automatically according to number of CPUs on the host system.'
This is discouraged by buildroot manual page. https://buildroot.org/downloads/manual/manual.html says: 'You should never use make -jN with Buildroot: top-level parallel make is currently not supported. Instead, use the BR2_JLEVEL option to tell Buildroot to run the compilation of each individual package with make -jN'