s390/boot: move EP_OFFSET and EP_STRING to head.S

Both macros are used only in decompressor's head.S, unnecessary to put
them in a global header used in many places like setup.h is.

Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>