kexec: split kexec_file syscall code to kexec_file.c

kexec: split kexec_file syscall code to kexec_file.c

Split kexec_file syscall related code to another file kernel/kexec_file.c
so that the #ifdef CONFIG_KEXEC_FILE in kexec.c can be dropped.

Sharing variables and functions are moved to kernel/kexec_internal.h per
suggestion from Vivek and Petr.

[[email protected]: fix bisectability]
[[email protected]: declare the various arch_kexec functions]
[[email protected]: fix build]
Signed-off-by: Dave Young <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Vivek Goyal <[email protected]>
Cc: Petr Tesarik <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Josh Boyer <[email protected]>
Cc: David Howells <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>