uboot-sign: support to add users specific image tree source

uboot-sign: support to add users specific image tree source

Currently, uboot-sign.bbclass only supports to create Image Tree Source(ITS)
for "u-boot" and "flat_dt". However, users may want to add their private
images into u-boot FIT image for specific application and purpose.

To make this bbclass more flexible and support to add users specific snippet
ITS, creates a new "UBOOT_FIT_USER_SETTINGS" variable. Users can add their
specific snippet ITS into this variable.

Example:

```
UBOOT_FIT_MY_ITS = '\
       myfw {\n\
            description = \"MY Firmware\";\n\
            data = /incbin/(\"myfw.bin\");\n\
            type = \"mytype\";\n\
            arch = \"myarch\";\n\
            os = \"myos\";\n\
            load = <0xb2000000>;\n\
            entry = <0xb2000000>;\n\
            compression = \"none\";\n\
        };\n\
'

UBOOT_FIT_USER_SETTINGS = "${UBOOT_FIT_MY_ITS}"
```

The generated ITS

```
       myfw {
            description = "My Firmware";
            data = /incbin/("myfw.bin");
            type = "mytype";
            arch = "myarch";
            os = "myos";
            load = <0xb2000000>;
            entry = <0xb2000000>;
            compression = "none";
       };
```

Add a variable "UBOOT_FIT_CONF_USER_LOADABLES" to load users specific images
and it is an empty by default.

Signed-off-by: Jamin Lin <jamin_lin@aspeedtech.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>