[yocto] Bitbake local.conf variable been override by weak assignment

[Rick Liu]

I have defined a global default value BRCM_NXT-RELEASE-216_BRANCH in local.conf,
and setup a weak assignment (?=) in my recipe,
but somehow the weak assignment override the local.conf global default value.
Does anyone have any idea?

build/conf/local.conf:
BRCM_NXT-RELEASE-216_BRANCH = "SIT_int_nxt_v216.1.15.0"
PREFERRED_VERSION_brcm-nxt-l2-drv = "nxtrelease%"

meta-brcm/meta-datacenter/recipes-kernel/nxt-l2-drv/brcm-nxt-l2drv_nxtrelese.bb:
BRCM_NXT-RELEASE-216_BRANCH ?= "RICK"

$ bitbake -e brcm-nxt-l2-drv
# $BRCM_NXT-RELEASE-216_BRANCH [2 operations]
#   set /home/rl891036/scrach/DHELPDESK-3176/poky/build/conf/local.conf:60
#     [_defaultval] "SIT_int_nxt_v216.1.15.0"
#   set? /home/rl891036/scrach/DHELPDESK-3176/poky/build/../meta-brcm/meta-datacenter/recipes-kernel/nxt-l2-drv/brcm-nxt-l2-drv_nxtrelease.bb:10
#     "RICK"
# pre-expansion value:
#   "RICK"
BRCM_NXT-RELEASE-216_BRANCH="RICK"