tools: Fix broken object compilation rules

As these rules depend on non-existing headers as well (likely copy &
pasted from fiptool), they never matched, and the built-in rules were
used. That led to random breakages when e.g. CPPFLAGS was suddenly
evaluated and contained invalid options.

For the stm32image, this reveals that we were relying on the built-in
rules by passing -D_GNU_SOURCE via CPPFLAGS, rather than using CFLAGS as
used in the local rule. Fix that as well.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>