Replace malloc and strcpy by strdup in
function parse_label_kernel.

[Backport of commit 51c5c28af59c ("cmd: pxe: use strdup to
 copy config")]

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Change-Id: I265e5843b2411596c17a0328e676c6e27ffc755d