Makefile: make u-boot-initial-env target depend explicitly on scripts_basic

Makefile: make u-boot-initial-env target depend explicitly on scripts_basic

We're seeing sporadic errors like

  ENVC    include/generated/env.txt
  HOSTCC  scripts/basic/fixdep
  ENVP    include/generated/env.in
  ENVT    include/generated/environment.h
  HOSTCC  tools/printinitialenv
/bin/sh: 1: scripts/basic/fixdep: not found
make[1]: *** [scripts/Makefile.host:95: tools/printinitialenv] Error 127
make[1]: *** Deleting file 'tools/printinitialenv'
make: *** [Makefile:2446: u-boot-initial-env] Error 2
make: *** Waiting for unfinished jobs....

where sometimes the "fixdep: not found" is instead "fixdep: Permission
denied" and the Error 127 becomes 126.

This smells like a race condition, and indeed it is: Currently,
u-boot-initial-env is a prerequisite of the envtools target, which
also lists scripts_basic as a prerequisite:

envtools: u-boot-initial-env scripts_basic $(version_h) $(timestamp_h) tools/version.h
	$(Q)$(MAKE) $(build)=tools/env

However, the u-boot-initial-env rule involves building the
printinitialenv helper, which in turn is built using an if_changed_dep
rule. That means we must ensure scripts/basic/fixdep is built and
ready before trying to build printinitialenv, i.e. the
u-boot-initial-env rule itself must depend on the phony scripts_basic
target.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Simon Glass <sjg@chromium.org>