emulators/linux_base-c7 ports

[Gleb Popov]

Hello.

It turned out that CentOS changed location where RPM's are hosted:
http://mirror.centos.org/centos/7.4.1708/readme

I tried changing

--- Mk/Uses/linux.mk    (revision 496462)
+++ Mk/Uses/linux.mk    (working copy)
@@ -47,7 +47,7 @@
 IGNORE=                        Linux CentOS ${LINUX_DIST_VER} is
unsupported on ${ARCH}
 .endif
 .elif ${linux_ARGS} == c7
-LINUX_DIST_VER?=       7.4.1708
+LINUX_DIST_VER?=       7

and it seem to mostly work, but many linux-c7 ports require a version bump
now. I can prepare a patch for that, but wanted to make sure I'm on a right
track first.