linux: clarify kernel configuration entries

Since BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE can either be a complete
.config file or a defconfig file, it can be confusing to the user
whether to choose BR2_LINUX_KERNEL_USE_DEFCONFIG or
BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG.

To avoid that confusion, clarify Kconfig entry messages for in-tree
defconfig and custom (def)config files.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>