Add feature to skip clearing SRAM in ROM.

This allows ROM builds to skip clearing SRAM with the feature `skip-clear-sram`. This can be useful for simulation builds where clearing SRAM takes a significant amount of time.

rom/dev/Cargo.toml

@@ -60,6 +60,7 @@ itrng = ["caliptra-hw-model/itrng"]
 verilator = ["caliptra-hw-model/verilator"]
 no-fmc = []
 fake-rom = []
+skip-clear-sram = []
 no-cfi = ["caliptra-image-verify/no-cfi", "caliptra-drivers/no-cfi"]
 slow_tests = []
 "hw-1.0" = ["caliptra-builder/hw-1.0", "caliptra-drivers/hw-1.0", "caliptra-registers/hw-1.0", "caliptra-hw-model/hw-1.0"]

rom/dev/src/start.S

@@ -104,6 +104,7 @@ _start:
     andi t1, t1, 0x3
     bne t1, x0, post_ecc_init
 
+#ifndef CARGO_FEATURE_SKIP_CLEAR_SRAM
     //
     // Cold Boot
     //
@@ -117,6 +118,7 @@ _start:
     la a0, DCCM_ORG         // dest
     la a1, DCCM_SIZE        // len 
     call _zero_mem256
+#endif // CARGO_FEATURE_SKIP_CLEAR_SRAM
 
 post_ecc_init:
 
@@ -354,9 +356,11 @@ exit_rom:
 
     // Save the FMC address
     addi a3, a0, 0
+#ifndef CARGO_FEATURE_SKIP_CLEAR_SRAM
     la a0, STACK_ORG         // dest
     la a1, STACK_SIZE        // len 
     call _zero_mem256
+#endif // CARGO_FEATURE_SKIP_CLEAR_SRAM
 
    // Clear all registers
    li x1,  0; li x2,  0; li x3,  0; li x4,  0;