Prebuilt Kernel Boot Failure Due to Insufficient INITRD_SIZE

[otteryc]

The current prebuilt kernel image and root filesystem (rootfs) include SDL support (see #551), which increases the space required for the initrd to load the rootfs (see #553). However, the default INITRD_SIZE (8MB, located in Makefile:55) is insufficient, causing the boot process to fail.

When building rv32emu using the command provided in README.md:

make ENABLE_SYSTEM=1 system

the system fails to boot due to the above issue.

00:19:16 INFO  src/riscv.c:422: Log level: TRACE
00:19:16 INFO  src/riscv.c:498: Kernel loaded
00:19:16 INFO  src/riscv.c:503: DTB loaded
[1]    660383 segmentation fault (core dumped)  ./rv32emu -k linux-image/Image -i linux-image/rootfs.cpio

However, explicitly setting INITRD_SIZE=32 allows the kernel to boot successfully:

make ENABLE_SYSTEM=1 INITRD_SIZE=32 system

Potential Solutions:

1. Increase the default INITRD_SIZE in Makefile to 32MB or larger.
2. Remove SDL support and DOOM-related files from default Linux image to reduce the required space.
3. Update the README to specify INITRD_SIZE=32 as the default for testing.

[ChinYikMing]

Potential Solutions:

1. Increase the default INITRD_SIZE in Makefile to 32MB or larger.
2. Remove SDL support and DOOM-related files from default Linux image to reduce the required space.
3. Update the README to specify INITRD_SIZE=32 as the default for testing.

The initrd image will gradually increase in size over time (requiring adjustments to INITRD_SIZE) until rv32emu supports booting with a block device. Once block device booting support is available, the initrd image can be removed, and the block device image can be used for booting instead.

Given this, I prefer the third option. In fact, the README already touches on this:
"Build with a larger INITRD_SIZE (e.g., 64 MiB) to run SDL-oriented applications, as the default 8 MiB is insufficient for SDL application artifacts." Therefore, the third option should also refine the description to clarify this point further.