Makefile: allow FMC_DRV in environment

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

Makefile

@@ -2,7 +2,7 @@
 # We have a problem here: this package may be a submodule of something
 # else (for example: fine-delay). If this is the case, fmc-bus is on ../
 # This external script returns the pathname of the "kernel" subdir of fmc-bus
-FMC_DRV = $(./check-fmc-bus)
+FMC_DRV ?= $(./check-fmc-bus)
 
 DIRS = $(FMC_DRV) kernel tools