driver: Use bash's default assignment operator for qemu_ram

qemu_ram only needs to be assigned to something different if the default
causes issues. Move the default assignment to its first use, which will
make adding new architectures this script easier as there is one less
variable to worry about.

[skip ci]

Signed-off-by: Nathan Chancellor <[email protected]>

Author: nathanchance

driver.sh

@@ -45,7 +45,6 @@ setup_variables() {
       config=multi_v5_defconfig
       image_name=zImage
       qemu="qemu-system-arm"
-      qemu_ram=512m
       qemu_cmdline=( -machine palmetto-bmc
                      -no-reboot
                      -dtb "${tree}/arch/arm/boot/dts/aspeed-bmc-opp-palmetto.dtb"
@@ -57,7 +56,6 @@ setup_variables() {
       config=aspeed_g5_defconfig
       image_name=zImage
       qemu="qemu-system-arm"
-      qemu_ram=512m
       qemu_cmdline=( -machine romulus-bmc
                      -no-reboot
                      -dtb "${tree}/arch/arm/boot/dts/aspeed-bmc-opp-romulus.dtb"
@@ -69,7 +67,6 @@ setup_variables() {
       config=multi_v7_defconfig
       image_name=zImage
       qemu="qemu-system-arm"
-      qemu_ram=512m
       qemu_cmdline=( -machine virt
                      -no-reboot
                      -drive "file=images/arm/rootfs.ext4,format=raw,id=rootfs,if=none"
@@ -85,7 +82,6 @@ setup_variables() {
       esac
       image_name=Image.gz
       qemu="qemu-system-aarch64"
-      qemu_ram=512m
       qemu_cmdline=( -cpu cortex-a57
                      -drive "file=images/arm64/rootfs.ext4,format=raw"
                      -append "console=ttyAMA0 root=/dev/vda" )
@@ -103,8 +99,7 @@ setup_variables() {
                          -append "console=ttyS0 root=/dev/sda" ) ;;
       esac
       image_name=bzImage
-      qemu="qemu-system-x86_64"
-      qemu_ram=512m ;;
+      qemu="qemu-system-x86_64" ;;
     "ppc32")
       config=ppc44x_defconfig
       image_name=zImage
@@ -272,7 +267,7 @@ boot_qemu() {
   local kernel_image=${tree}/arch/${ARCH}/boot/${image_name}
   test -e ${kernel_image}
   qemu=( timeout 2m unbuffer "${qemu}"
-                             -m "${qemu_ram}"
+                             -m "${qemu_ram:=512m}"
                              "${qemu_cmdline[@]}"
                              -nographic
                              -kernel "${kernel_image}" )