build: fix pkg-config detection when inside of a nix-shell

The output of command_all when inside a nix-shell looks like the below
where /usr does not match:

 ➜ scripts/command_all.sh pkg-config
/nix/store/ifr6srqgpvygd5vp14748d109ri31isv-pkg-config-wrapper-0.29.2/bin/pkg-config

Signed-off-by: Sandro Jäckel <[email protected]>

Makefile

@@ -14,7 +14,7 @@ $(if $(findstring $(space),$(TOPDIR)),$(error ERROR: The path to the OpenWrt dir
 
 world:
 
-DISTRO_PKG_CONFIG:=$(shell $(TOPDIR)/scripts/command_all.sh pkg-config | grep '/usr' -m 1)
+DISTRO_PKG_CONFIG:=$(shell $(TOPDIR)/scripts/command_all.sh pkg-config | grep -e '/usr' -e '/nix/store' -m 1)
 
 export ORIG_PATH:=$(if $(ORIG_PATH),$(ORIG_PATH),$(PATH))
 export PATH:=$(if $(STAGING_DIR),$(abspath $(STAGING_DIR)/../host/bin),$(TOPDIR)/staging_dir/host/bin):$(PATH)