x86/boot/e820: Rename the e820_table_firmware to e820_table_kexec

x86/boot/e820: Rename the e820_table_firmware to e820_table_kexec

Currently the e820_table_firmware[] table is mainly used by the kexec,
and it is not what it's supposed to be - despite its name it might be
modified by the kernel.

So change its name to e820_table_kexec[]. In the next patch we will
introduce the real e820_table_firmware[] table.

No functional change.

Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Xunlei Pang <xlpang@redhat.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>