devicemodels/spi_sdcard: implement more commands as used by u-boot.
tests: fixup linker script formatting.
tests: work around bad segment in timer_irq test.

Even though there was only .text.sdram which should go into the sdram
MEMORY section (0x20000000 + 32MB), binutils was generating a PT_LOAD
segment with a virtual/physical address of 0x1fffff80 and p_offset of 0
so would get loaded into an invalid address.
debugger/loadelf: load segments rather than sections.

When loading an executable we should really be loading segments rather
than sections.