#### Config attempt #2 ####

meta-neotion$ grep IMAGE_FEATURES conf/distro/neotion.conf recipes-core/images/neotion-ottgw-*
conf/distro/neotion.conf:IMAGE_FEATURES[validitems] += "nfsroot swupdate"
recipes-core/images/neotion-ottgw-dev.bb:IMAGE_FEATURES += "swupdate"
recipes-core/images/neotion-ottgw-prod.bb:IMAGE_FEATURES += "nfsroot"

#### Result ####
Same process: do_cleanall on recipe that rely on nfsroot and the bitbake <my whole image> with verbose output:

$ grep IMAGE_FEA output.log
u-boot-fw-utils-v2015.01+gitAUTOINC+2fcbc68e05-r0 do_install: + bbwarn IMAGE_FEATURES =  debug-tweaks
+ printf %b\0 bbwarn IMAGE_FEATURES =  debug-tweaks
+ bbwarn nfsroot is NOT set in IMAGE_FEATURES, leaving boot.init untouched...
+ printf %b\0 bbwarn nfsroot is NOT set in IMAGE_FEATURES, leaving boot.init untouched...
WARNING: u-boot-fw-utils-v2015.01+gitAUTOINC+2fcbc68e05-r0 do_install: IMAGE_FEATURES =  debug-tweaks
WARNING: u-boot-fw-utils-v2015.01+gitAUTOINC+2fcbc68e05-r0 do_install: nfsroot is NOT set in IMAGE_FEATURES, leaving boot.init untouched...