This patch makes iscsi-boot element support not only just
DISTRO_NAME centos7 but also centos and centos-minimal.
Change-Id: I8db8b01f35b2e572666badd8d2316d24a5e4287a
in CentOS build case building an image with "iscsi-boot" and "dracut-regenerate" will exit building because of statement "[ "$found" = 0 ]"
Change-Id: I1a6d60e9ec5f5cb508866c8376465c3e73551a30
There is an use of get_image_element_array on the environment.d
phase, for the iscsi-boot element.
This function is not available on that step. So moving the check
at next step, extra-data-d, where it is available.
Change-Id: I89cfe565492142c2f7962109360fcbcebadfd469