treewide: use /usr/bin/env where appropriate

Some Unix systems (GuixSD, NixOS) do not install programs like
Bash and Python to /usr/bin, and /usr/bin/env has to be used to
locate these instead.

Change-Id: I7546bcb881c532adc984577ecb0ee2ec4f2efe00
Signed-off-by: Yegor Timoshenko <yegortimoshenko@riseup.net>
Reviewed-on: https://review.coreboot.org/28953
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/cbmem/Makefile

@@ -16,7 +16,7 @@
 PROGRAM   = cbmem
 ROOT      = ../../src
 CC       ?= $(CROSS_COMPILE)gcc
-INSTALL  ?= /usr/bin/install
+INSTALL  ?= /usr/bin/env install
 PREFIX   ?= /usr/local
 CFLAGS   ?= -O2
 CFLAGS   += -Wall -Werror