Revert "build: replace which with Bash command built-in"

This reverts commit c7aec47e5e.

The original commit replaces 'which' with 'command'. Sadly most of
them are not equivalent and for 'which -a', there is no easy
replacements that would not reimplement PATH parsing logic. Hence
revert. Keeping a dependency on which is absolutely fine.

Signed-off-by: Clemens Fruhwirth <clemens@endorphin.org>

Makefile

@@ -14,7 +14,7 @@ $(if $(findstring $(space),$(TOPDIR)),$(error ERROR: The path to the OpenWrt dir
 
 world:
 
-DISTRO_PKG_CONFIG:=$(shell command -pv pkg-config | grep -E '\/usr' | head -n 1)
+DISTRO_PKG_CONFIG:=$(shell which -a pkg-config | grep -E '\/usr' | head -n 1)
 export PATH:=$(TOPDIR)/staging_dir/host/bin:$(PATH)
 
 ifneq ($(OPENWRT_BUILD),1)