doc: Add manual bitbake build preparation documentation

This patch clarifies that the kas build tool and provided kas configuration YAML
files are the recommended approach to building EWAOL images, but adds additional
documentation to briefly describe the necessary steps for preparing and building
an image without kas.

Issue-Id: SCM-3318
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I2ca57786ad3b2ddbab6e9450a1477f31315a2cba