microblaze: use built-in function to get CPU_{MAJOR,MINOR,REV}

Use built-in functions instead of shell commands to avoid forking
processes.

Signed-off-by: Masahiro Yamada <[e-mail]>
Reviewed-by: Nicolas Schier <[e-mail]>

arch/microblaze/Makefile

···
 # What CPU version are we building for, and crack it open
 # as major.minor.rev
 CPU_VER   := $(CONFIG_XILINX_MICROBLAZE0_HW_VER)
-CPU_MAJOR := $(shell echo $(CPU_VER) | cut -d '.' -f 1)
-CPU_MINOR := $(shell echo $(CPU_VER) | cut -d '.' -f 2)
-CPU_REV   := $(shell echo $(CPU_VER) | cut -d '.' -f 3)
+CPU_MAJOR := $(word 1, $(subst ., , $(CPU_VER)))
+CPU_MINOR := $(word 2, $(subst ., , $(CPU_VER)))
+CPU_REV   := $(word 3, $(subst ., , $(CPU_VER)))
 
 export CPU_VER CPU_MAJOR CPU_MINOR CPU_REV