linux-fslc[-imx]: use KBRANCH instead of SRCBRANCH in kernel build

linux-fslc[-imx]: use KBRANCH instead of SRCBRANCH in kernel build

Kernel recipe should define a kernel branch used in recipe via KBRANCH
and not a customized variable SRCBRANCH.

KBRANCH variable is used by several OE tools (e.g. devtool) to define
the checkout policy when recipe is modified to a separate user
workspace.

Replace the occurence of SRCBRANCH with KBRANCH in kernel recipes, and
change fetcher URI to use KBRANCH variable for checkout.

Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>