dahdi-linux: fix Module.symvers handling

dahdi-linux: fix Module.symvers handling

This sets PKG_EXTMOD_SUBDIRS so kernel.mk can find the DAHDI
Module.symvers file.

Also, this puts KERNEL_MAKE_FLAGS into the make flags. This way
Module.symvers files of other modules are made available, plus there is
no need anymore to specify ARCH and CROSS_COMPILE, as KERNEL_MAKE_FLAGS
already takes care of that.

TARGET_CONFIGURE_OPTS is dropped because this seems out of place
(configure vs. compile).

Signed-off-by: Sebastian Kemper <[email protected]>