tools: k3_get_x509 cert: Add a script to generate x509 certificate for K3 devices

TI's K3 boot architecture mandates a x509 certificate for every boot image.
While signing the image K3 ROM allows for two types of keys based on which
the boot image gets loaded in different ways:
- Degenerate RSA keys: This generates a signature which is equal to the digest.
		       When ROM sees this, it does a DMA for copying the images,
		       which significantly improves the boot time.
- Any other key: Does a memcpy to load the image. This is introduced as a
		 fallback for DMA copy.

Add a script for generating boot images with the above options. Default
generates image using rsa degenerate key in order to improve boot time.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>