gccarmemb: don't assume 'dtc' is in /usr/bin

Just search for it in the system PATH.

Issue: ZEP-2211
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/scripts/Makefile.toolchain.gccarmemb b/scripts/Makefile.toolchain.gccarmemb
index 4e7db43..f5055ef 100644
--- a/scripts/Makefile.toolchain.gccarmemb
+++ b/scripts/Makefile.toolchain.gccarmemb
@@ -10,6 +10,6 @@
 LIB_INCLUDE_DIR += -L ${LIBGCC_DIR}
 TOOLCHAIN_CFLAGS = -I${GCCARMEMB_TOOLCHAIN_PATH}/arm-none-eabi/include
 
-DTC ?= /usr/bin/dtc
+DTC ?= dtc
 
 export CROSS_COMPILE TOOLCHAIN_LIBS TOOLCHAIN_CFLAGS LIB_INCLUDE_DIR DTC