kbuild: Fix reading of .config in link-vmlinux.sh

The shell '.' command is not required to search the current directory as
a fallback and in fact newer versions of bash in sh-mode do not do this.
Force reading the file from the current directory if $KCONFIG_CONFIG
contains no '/'.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michal Marek <mmarek@suse.cz>