Makefile: Add default for HOSTPKG_CONFIG

Kconfig uses this variable to detect `ncurses` compilation and
linking flags. Without it, some guesswork fallback is assumed
that only works by chance.

Change-Id: Iad21bdb2d61db04cf7397ab447c7c045e2067705
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70584
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Büchler <michael.buechler@posteo.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Makefile

@@ -69,6 +69,9 @@ endif
 HOSTCFLAGS := -g
 HOSTCXXFLAGS := -g
 
+HOSTPKG_CONFIG ?= pkg-config
+COREBOOT_EXPORTS += HOSTPKG_CONFIG
+
 PREPROCESS_ONLY := -E -P -x assembler-with-cpp -undef -I .
 
 export $(COREBOOT_EXPORTS)