Makefile: Make pkg-config mandatory to find libusb1

Use `make HAS_LIBUSB1=yes/no` to override the pkg-config detection and
`CONFIG_LIBUSB1_CFLAGS` and `CONFIG_LIBUSB1_LDFLAGS` to set cflags and
ldflags manually.

Change-Id: I4f24be647d25dde24c41514f8964b7134205867c
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/59049
Original-Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72263
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
2 files changed