mknxflat is built from tools/nxflat by the NuttX build itself, so the
toolchain section no longer sends the reader to buildroot for it; only
ldnxflat still comes from there. Bring the mknxflat usage text in line
with the tool, note that MKNXFLAT and LDNXFLAT are supplied by the ARM
Toolchain.defs, and correct the r10 references left in the PIC
descriptions.
Describe the module ABI marker, so that a user whose prebuilt module
starts failing exec() with ENOEXEC finds out that the loader refuses a
module whose import table does not name __nxflat_abi_v2, and that
rebuilding the module is the fix.
Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>