build: replace which with Bash command built-in

`which` utility is not shipped by default for example on recent Arch
Linux and then any steps relying on its presence fails, like for example
following Python3 prereq build check:

 $ python3 --version
 Python 3.9.1

 $ make
 /bin/sh: line 1: which: command not found
 /bin/sh: line 1: which: command not found
 /bin/sh: line 1: which: command not found
 ...
 Checking 'python3'... failed.
 ...

Fix this by switching to Bash builtin `command` which should provide
same functionality.

Fixes: FS#3525
Signed-off-by: Petr Štetiar <ynezz@true.cz>

include/prereq.mk

@@ -52,7 +52,7 @@ endef
 
 define RequireCommand
   define Require/$(1)
-    which $(1)
+    command -pv $(1)
   endef
 
   $$(eval $$(call Require,$(1),$(2)))
@@ -106,7 +106,7 @@ define SetupHostCommand
 	           $(call QuoteHostCommand,$(11)) $(call QuoteHostCommand,$(12)); do \
 		if [ -n "$$$$$$$$cmd" ]; then \
 			bin="$$$$$$$$(PATH="$(subst $(space),:,$(filter-out $(STAGING_DIR_HOST)/%,$(subst :,$(space),$(PATH))))" \
-				which "$$$$$$$${cmd%% *}")"; \
+				command -pv "$$$$$$$${cmd%% *}")"; \
 			if [ -x "$$$$$$$$bin" ] && eval "$$$$$$$$cmd" >/dev/null 2>/dev/null; then \
 				mkdir -p "$(STAGING_DIR_HOST)/bin"; \
 				ln -sf "$$$$$$$$bin" "$(STAGING_DIR_HOST)/bin/$(strip $(1))"; \