docs/manual: document the new kconfig-package variable

The previous patch introduced the new FOO_KCONFIG_DEFCONFIG variable
to specify a defconfig rule rather than a (def)config file.

Add this to the manual.

Also document the pre-existing FOO_KCONFIG_FILE for which the
explanations were missing altogether.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>