kernel/linux: use a custom kernel source directory

Allows using either a tarball or a directory as the custom kernel
source location.

Signed-off-by: Vincent BENOIT <sinseman44@gmail.com>
[yann.morin.1998@anciens.enib.fr: fix space damage, detailed commit message]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Default test suite configuration

 # Default DejaGnu configuration

 DG_TOOLNAME = gcc

 DG_TARGET_HOSTNAME = 127.0.0.1

 DG_TARGET_USERNAME = root

 # Default tests

 DG_C_TESTS = 

 DG_CPP_TESTS =