zipl/boot/Makefile: fix failure to build, do not specify .lds twice.
zipl/boot/Makefile: fix failure to build, do not specify .lds twice.

Whilst stage3.lds is a dependency, it is specified via SFLAGS and must
not be repeated again along all the .o files. Filter anything but .o
files.

Fixes failure to build from source on Ubuntu 19.10.

Bug-Ubuntu: https://bugs.launchpad.net/ubuntu/+source/s390-tools/+bug/1833238
Closes: https://github.com/ibm-s390-tools/s390-tools/pull/63
Signed-off-by: Dimitri John Ledkov <446b8b5d376e3ab2e6b2d5d0d93dc2d3de728b2c@ubuntu.com>
Acked-by: Stefan Haberland <dec981e3bbb165d021029c42291faf06f59827c1@linux.ibm.com>
Signed-off-by: Jan Höppner <96138615ab4af0813785fa10d283f3d37eca3b78@linux.ibm.com>

ibm-s390-tools/s390-tools,ibm-s390-tools/s390-tools,ibm-s390-tools/s390-tools,ibm-s390-tools/s390-tools