base-files: nand: use CI_KERNPART whenever the kernel volume is needed

base-files: nand: use CI_KERNPART whenever the kernel volume is needed

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This patch is in continuation of: commit 93aa86040523
"procd: nand: make it possible to configure kernel and ubi partition"

The $CI_KERNPART variable should be used in place
of the fixed "kernel" partition name. This allows
targets to specifiy alternate names for the kernel
partition.

Cc: Chris Blake <chrisrblake93@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>