cbfstool: Make add-stage support multiple loadable segments

For x86 eXecute-In-Place (XIP) pre-memory `.data` section support, we
have to use an extra segment as the VMA/LMA of the data is different
than the VMA/LMA of the code.

To support this requirement, this patch makes cbfstool:
1. Allow the load of an ELF with an extra segment
2. Makes add-stage for XIP (cf. parse_elf_to_xip_stage()) write its
   content to the output binary.

To prevent the creation of unsuitable binaries, cbfstool verifies that
the LMA addresses of the segments are consecutives.

TEST=XIP pre-memory stages with a `.data` section have the `.data`
     section covered by a second segment properly included right after
     the code.

Change-Id: I480b4b047546c8aa4e12dfb688e0299f80283234
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77584
Reviewed-by: Julius Werner <jwerner@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/cbfstool/rmodule.h

@@ -29,6 +29,8 @@ struct rmod_context {
 	struct parsed_elf pelf;
 	/* Program segment. */
 	Elf64_Phdr *phdr;
+	/* Number of loadable segments. */
+	size_t nsegments;
 	/* Symbol string table. */
 	char *strtab;