sencha/shodan: allow boot_rom pathname to be overridden

Bypass-Presubmit-Reason: missing bazelversion

Change-Id: I7ccf2e15fc80f22715e5ef86fde15d8159152d56

sencha.resc

diff --git a/sencha.resc b/sencha.resc
index 2dd0624..308605b 100644
--- a/sencha.resc
+++ b/sencha.resc
@@ -50,7 +50,8 @@
 uart5-analyzer TimestampFormat None
 
 # Load the boot rom into the 32k rom at 0x8000 (useVirtualAddress = false, allowLoadsOnlyToMemory = true)
-sysbus LoadELF @out/sencha/hw/boot_rom.elf false true cpu0
+$boot_rom ?= @out/sencha/hw/boot_rom.elf
+sysbus LoadELF $boot_rom false true cpu0
 
 # Start cpu0 at the bootrom reset vector, which is stored immediately after the
 # bootrom interrupt vector table at 0x8080.

shodan.resc

diff --git a/shodan.resc b/shodan.resc
index d83a718..cfad367 100644
--- a/shodan.resc
+++ b/shodan.resc
@@ -58,7 +58,8 @@
 uart5-analyzer TimestampFormat None
 
 # Load the boot rom into the 32k rom at 0x8000 (useVirtualAddress = false, allowLoadsOnlyToMemory = true)
-sysbus LoadELF @out/matcha/hw/boot_rom.elf false true cpu0
+$boot_rom ?= @out/matcha/hw/boot_rom.elf
+sysbus LoadELF $boot_rom false true cpu0
 
 # Load the tarball of CAmkES components, cpio bundle of built-in files,
 # the seL4 kernel, and preload the secure core binary.