fw/Makefile: add explicit path to CROSS

Instead of trying to modify PATH, add an explicit directory name to
CROSS.

Makefile

@@ -1,5 +1,3 @@
-export PATH := $(CURDIR)/tools/gnu:$(PATH)
-
 SUBDIRS = tools fw fpga
 
 all clean spotless :

fw/Makefile

@@ -1,7 +1,6 @@
 MAKEFLAGS += -R -r
-export PATH := $(CURDIR)/tools/gnu/bin:$(PATH)
 
-CROSS	  = riscv32-unknown-elf-
+CROSS	  = ../tools/gnu/bin/riscv32-unknown-elf-
 CC	  = $(CROSS)gcc
 LD	  = $(CROSS)ld
 OBJCOPY   = $(CROSS)objcopy

tools/Makefile

@@ -1,7 +1,6 @@
 MAKEFLAGS += -R -r
 
 prefix   := $(CURDIR)
-export PATH := $(prefix)/gnu/bin:$(PATH)
 
 tooldir   = riscv-gnu-toolchain
 binutils  = $(tooldir)/riscv-binutils