ci: make bootstrap just another kas overlay

Simplify the gitlab-ci by having the bootstrap build as just another Kas
overlay file, so there's no duplication of build script.

Change-Id: I7341750d2ae7f3c146bfe323f61fa98c0f3121c0
Signed-off-by: Ross Burton <ross.burton@arm.com>

.gitlab-ci.yml

@@ -36,13 +36,9 @@ stages:
 
 # Build a number of native tools first to ensure the other builders don't race
 # over them
-bootstrap:
+n1sdp/bootstrap:
   extends: .build
   stage: bootstrap
-  variables:
-    KAS_TARGET: binutils-cross-aarch64 gcc-cross-aarch64 python3-native opkg-native rpm-native
-  script:
-  - kas build kas/n1sdp.yml
 
 # What percentage of machines in the layer do we build
 machine-coverage:

kas/bootstrap.yml

@@ -0,0 +1,9 @@
+header:
+  version: 9
+
+target:
+  - binutils-cross-aarch64
+  - gcc-cross-aarch64
+  - python3-native
+  - opkg-native
+  - rpm-native