Add qemu environment variable to dump device tree.

Use make DUMPDTB=1 qemu to dump the device tree.

Signed-off-by: KieranMusser <59939188+KieranMusser@users.noreply.github.com>

Author: KieranMusser

src/targets/riscv64/qemu-riscv64.mak

@@ -3,7 +3,12 @@
 # SPDX-License-Identifier: GPL-3.0-or-later
 
 kernel-cflags += -march=rv64imafdcbv_zicsr_zicntr_zihpm_ziccif_ziccrse_ziccamoa_zicclsm_zic64b_za64rs_zihintpause_zba_zbb_zbs_zicbom_zicbop_zicboz_zfhmin_zkt_zvfhmin_zvbb_zvkt_zihintntl_zicond_zimop_zcmop_zcb_zfa_zawrs_svpbmt_svinval_svnapot_sstc_sscofpmf_zifencei
+
+ifdef DUMPDTS
+target-qemuflags = --machine virt,dumpdtb=qemu.dtb --cpu rva23s64 --smp 2 -m 1G --netdev user,id=net0 -device rtl8139,netdev=net0,bus=pcie.0
+else
 target-qemuflags = --machine virt --cpu rva23s64 --smp 2 -m 1G --netdev user,id=net0 -device rtl8139,netdev=net0,bus=pcie.0
+endif
 
 ifdef NETDUMP
 target-qemuflags += -object filter-dump,id=net0,netdev=net0,file=net_dump.pcap