kbuild: parameterize the .o part of suffix-search

The suffix-search macro hard-codes the suffix, '.o'.

Make it a parameter so that the multi-search and real-search macros
can be reused for foo-dtbs syntax introduced by commit 15d16d6d
("kbuild: Add generic rule to apply fdtoverlay").

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>