fitimage: add support to build arbitrary FIT images

The FIT image support in OE is quite limited:
 1) No support to build an arbitrary number of FIT images since the FIT
    image generation is tightly coupled to the kernel image.
 2) A lot of U_BOOT-specific variables which may not be necessary for
    other bootloaders.
 3) No usage of the meta-oe signing.bbclass for signed FIT images.

This alternative class is added to solve the above-mentioned problems:
 1) The class can be inherited by an arbitrary number of
    <fit-image-name>.bb recipes to generate FIT images
 2) No U_BOOT-specific variables are used
 3) <fit-image-name>.bb recipes can prepend the do_fitimage() to
    provide the key using the signing.bbclass e.g.:

    do_fitimage:prepend() {
        signing_prepare
        signing_use_role "${FITIMAGE_SIGNING_KEY_ROLE}"
    }

    Then enable and configure signing as follows:

    FITIMAGE_SIGN = "1"
    FITIMAGE_MKIMAGE_EXTRA_ARGS = "--engine pkcs11"
    FITIMAGE_SIGN_KEYDIR = "${PKCS11_URI}

This class is inspired by the meta-phytec fitimage.bbclass [1].

[1] https://git.phytec.de/meta-phytec/tree/classes/fitimage.bbclass

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Signed-off-by: Khem Raj <raj.khem@gmail.com>

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