build: add buildbot specific config option for setting defaults

This can be used to tweak the buildbot behavior without having to change
buildbot's configuration.
It will also allow us to add more aggressive clean steps (e.g. on
toolchain changes), which would break developers' workflows if enable
by default.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

config/Config-build.in

@@ -9,7 +9,7 @@ menu "Global build settings"
 
 	config ALL_NONSHARED
 		bool "Select all target specific packages by default"
-		default ALL
+		default ALL || BUILDBOT
 
 	config ALL_KMODS
 		bool "Select all kernel module packages by default"
@@ -19,6 +19,16 @@ menu "Global build settings"
 		bool "Select all userspace packages by default"
 		default n
 
+	config BUILDBOT
+		bool "Set build defaults for automatic builds (e.g. via buildbot)"
+		default n
+		help
+		  This option changes several defaults to be more suitable for
+		  automatic builds. This includes the following changes:
+		  - Deleting build directories after compiling (to save space)
+		  - Enabling per-device rootfs support
+		  ...
+
 	config SIGNED_PACKAGES
 		bool "Cryptographically signed package lists"
 		default y
@@ -61,7 +71,7 @@ menu "Global build settings"
 		bool
 		prompt "Collect kernel debug information"
 		select KERNEL_DEBUG_INFO
-		default n
+		default BUILDBOT
 		help
 		  This collects debugging symbols from the kernel and all compiled modules.
 		  Useful for release builds, so that kernel issues can be debugged offline