[OE-core] [PATCH 1/2] bitbake.conf: use weaker assignment for IMAGE_OVERHEAD_FACTOR

[Qi.Chen at windriver.com]

From: Chen Qi <Qi.Chen at windriver.com>

This allows image recipes to set a default value and still provides
an option for users to override it.

For example, we set a default value in core-image-sato.bb.
    IMAGE_OVERHEAD_FACTOR ?= "2.0"
And users who build sato image could still override it in their distro
conf files or local.conf.

[YOCTO #4109]

Signed-off-by: Chen Qi <Qi.Chen at windriver.com>
---
 meta/conf/bitbake.conf |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/meta/conf/bitbake.conf b/meta/conf/bitbake.conf
index cd5d61e..6d947d8 100644
--- a/meta/conf/bitbake.conf
+++ b/meta/conf/bitbake.conf
@@ -412,7 +412,7 @@ IMAGE_LINK_NAME = "${IMAGE_BASENAME}-${MACHINE}"
 
 # This option allows for a percentage overage of the actual image size rather than a
 # fixed extra space, this is space needed for initial startup and basic operations.
-IMAGE_OVERHEAD_FACTOR ?= "1.3"
+IMAGE_OVERHEAD_FACTOR ??= "1.3"
 # This option allows for adding additional space in K above and beyond what the 
 # IMAGE_OVERHEAD_FACTOR might add. This space is for additional packages, user data, ...
 # To set a fixed size then overriding IMAGE_ROOTFS_SIZE with the max size one wants
-- 
1.7.9.5