[source] sdk: provide a config symbol for detecting builds within the SDK

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/?p=source.git;a=commitdiff;h=12703d5b294a00fd6a443dae82a0d05f7954b320

commit 12703d5b294a00fd6a443dae82a0d05f7954b320
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Sun Jul 31 12:14:38 2016 +0200

    sdk: provide a config symbol for detecting builds within the SDK
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 target/sdk/files/Config.in | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/target/sdk/files/Config.in b/target/sdk/files/Config.in
index 064cb05..0614f31 100644
--- a/target/sdk/files/Config.in
+++ b/target/sdk/files/Config.in
@@ -18,6 +18,10 @@ menu "Global build settings"
 
 endmenu
 
+config IN_SDK
+	default y
+	bool
+
 config MODULES
 	bool
 	default y