doc: environment: Further expand on Image locations and provide example

Start by elaborating on what some of our constraints tend to be with
image location values, and document where these external constraints
can come from.  Provide a new subsection, an example based on the TI
ARMv7 OMAP2PLUS families of chips, that gives sample values and explains
why we use these particular values.  This is based on what is in
include/configs/ti_armv7_common.h as of fb3ad9bd ("TI: Add, use a
DEFAULT_LINUX_BOOT_ENV environment string") as this contains just the
values referenced in this document now and not some of the further
additions that are less generic.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>