# cat /disks/temp/etc/yum.repos.d/c7buildroot.repo
[c7buildroot]
name=c7buildroot
baseurl=http://armv7.dev.centos.org/repodir/c7-buildroot/
#baseurl=ftp://192.168.1.3/c7buildroot
gpgcheck=0
enabled=1

# cat /disks/temp/etc/yum.repos.d/c7pass1.repo
[c7pass1]
name=c7pass1
baseurl=http://armv7.dev.centos.org/repodir/c7-pass-1/
#baseurl=ftp://192.168.1.3/c7pass1/
gpgcheck=0
enabled=1

# yum --installroot=/disks/temp/ groupinstall @core
There is no installed groups file.
Maybe run: yum groups mark convert (see man yum)
c7buildroot                                                 | 2.9 kB  00:00
c7pass1                                                     | 2.9 kB  00:00
(1/2): c7buildroot/primary_db                               | 2.3 MB  00:05
(2/2): c7pass1/primary_db                                   | 2.8 MB  00:09
Warning: group core does not exist.
Maybe run: yum groups mark install (see man yum)
Error: No packages in any requested group available to install or update