uboot-sign: set load address and entrypoint

uboot-sign: set load address and entrypoint

According to the design of uboot-sign.bbclass and kernel-fitimage.bbclass,
both of them use an UBOOT_LOADADDRESS variable to set the load address of
kernel and u-boot image and use an UBOOT_ENTRYPOINT variable to set the entry address of
kernel and u-boot image.

However, users may want to set the different load address of u-boot
and kernel image. Therefore, adds UBOOT_FIT_UBOOT_LOADADDRESS
and UBOOT_FIT_UBOOT_ENTRYPOINT variables to set the
load address and entry point of u-boot image, respectively.

The value of UBOOT_FIT_UBOOT_ENTRYPOINT is UBOOT_LOADADDRESS by default.
The value of UBOOT_FIT_UBOOT_ENTRYPOINT is UBOOT_ENTRYPOINT by default.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>