Allow setting non-default padding

[ZhekaS]

Currently the padding is set solely based on the architecture assumptions:

elf2tab/src/convert.rs

Lines 192 to 204 in 06b05fd

	// Add trailing padding for certain architectures.
	//
	// - ARM: make sure the entire TBF is a power of 2 to make configuring the
	// MPU easy.
	// - RISC_V: make sure the entire TBF is a multiple of 4 to meet TBF
	// alignment requirements.
	// - x86: use 4k padding to match page size.
	let trailing_padding = match elf_file.ehdr.e_machine {
	elf::abi::EM_ARM => Some(TrailingPadding::TotalSizePowerOfTwo),
	elf::abi::EM_RISCV => Some(TrailingPadding::TotalSizeMultiple(4)),
	elf::abi::EM_386 => Some(TrailingPadding::TotalSizeMultiple(4096)),
	_ => None,
	};

However these assumptions are not always correct. For instance, in case of RISC-V, the MPU (PMP) might have greater alignment requirements (see tock/tock#4417). This can be kind of worked around by adding something like

    .dummy : {
        . = ALIGN(4096); /* For 4096 granularity */
    } > FLASH

but it also breaks as elf2tbf adds at least 4 bytes for relocation data to the total size.