kernel-fitimage: allow overriding FIT configuration prefix

kernel-fitimage: allow overriding FIT configuration prefix

The prefix was recently changed to play ball with the CVE-2021-27138
workaround in U-Boot. Already deployed bootloaders though may still
expect a configuration name containing @ or they may not be affected
by this issue.

Also, uses may want to customize it beyond the [@-] issue: When
device trees are built from a recipe using kernel-devicetree.bbclass
the configuration nodes will contain the parent directories, e.g.

KERNEL_DEVICETREE = "freescale/imx8mm-myboard.dtb"

will become "conf-freescale_imx8mm-myboard.dtb". When moving to
devicetree.bbclass, this prefix go away. With this change here,
users can restore it easily by setting FIT_CONF_PREFIX =
"conf-freescale_", ensuring backwards compatibility.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>