Building with the attached random configuration file,

  LD      init/built-in.o
crypto/built-in.o: In function `RSA_verify_signature':
rsa.c:(.text+0x2dc5e): undefined reference to `mpi_get_nbits'
rsa.c:(.text+0x2dc68): undefined reference to `mpi_get_nbits'
rsa.c:(.text+0x2dc89): undefined reference to `mpi_free'
rsa.c:(.text+0x2dca4): undefined reference to `mpi_cmp_ui'
rsa.c:(.text+0x2dcb7): undefined reference to `mpi_cmp'
rsa.c:(.text+0x2dcc6): undefined reference to `mpi_alloc'
rsa.c:(.text+0x2dce7): undefined reference to `mpi_powm'
rsa.c:(.text+0x2dcff): undefined reference to `mpi_get_nbits'
rsa.c:(.text+0x2dd1c): undefined reference to `mpi_get_buffer'
rsa.c:(.text+0x2de07): undefined reference to `mpi_free'
make: *** [vmlinux] Error 1