Replace @RUNTIME_NAME@ with the target in generated files

In commit 966bd57 for PR kata-containers#902, the makefile was changed to automate
the replacement of user variables. However, one variable was treated
specially in the original `sed` replacements, namely `RUNTIME_NAME`
which was replaced by `$(TARGET)`.

This commit adds the `RUNTIME_NAME` variable to the makefile in order
to ensure that the replacement works correctly.

Fixes: kata-containers#993

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

src/runtime/Makefile

@@ -47,6 +47,9 @@ PROJECT_ORG = $(PROJECT_TAG)
 PROJECT_URL = https://github.com/$(PROJECT_ORG)
 PROJECT_BUG_URL = $(PROJECT_URL)/kata-containers/issues/new
 
+# @RUNTIME_NAME@ should be replaced with the target in generated files
+RUNTIME_NAME = $(TARGET)
+
 # list of scripts to install
 SCRIPTS :=