[LEDE-DEV] [RFC v2 1/3] include: image.mk: make ext4 reserved blocks percentage optional

[Jo-Philipp Wich]

Hi Michael,

the line of thinking here was that the Image/mkfs/ext4 macro might get
used in other target image build steps in which case there might not be
CONFIG_TARGET_EXT4_RESERVED_PCT symbol available since it is bound to
TARGET_ROOTFS_EXT4FS.

In any case the change will neither alter current behavior nor reduce
functionality but it allows reusing Image/mkfs/ext4 elsewhere.

~ Jo