fetch2/perforce: Rework to support SRCREV and P4CONFIG

fetch2/perforce: Rework to support SRCREV and P4CONFIG

In recipes which use the perforce fetcher, enable use of SRCREV to
specify any of: ${AUTOREV}, changelist number, p4date, or label.  This
is more in-line with how the other fetchers work for source control
systems.

Allow p4 to use the P4CONFIG env variable to define the server URL,
username, and password if not provided in a recipe.

This does change existing perforce fetcher usage by recipes and will
likely need those recipes which use the perforce fetcher to be updated.
No recipes in oe-core use the perforce fetcher.

References [YOCTO #6303]

Signed-off-by: Andrew Bradford <andrew.bradford@kodakalaris.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>