image: Update image_decomp() to avoid ifdefs

Adjust this function so that preprocessor macros are not needed. With
this, the host build uses more of the same header files as the target
build.

Rather than definining CONFIG_SYS_MALLOC_LEN, add a CONSERVE_MEMORY
define, since that is the purpose of the value.

This appears to have no impact on code size from a spot check of a few
boards (snow, firefly-rk3288, boston32r2el, m53menlo).

Signed-off-by: Simon Glass <sjg@chromium.org>