Gracefully handle 64-bit signed-extended 32-bit Load addresses

To follow the MIPS 32-bit and 64-bit memory map conventions (*) recent
MIPS Linux kernels are using a 64-bit sign extended value
(0xffffffff80010000) for the 32-bit load address (0x80010000) of the
Creator CI20 board kernel.  When this 64-bit argument was passed to
mkimage running on a 32-bit machine such as the Creator CI20 board the
load address was incorrectly formed from the upper 32-bit sign-extend
bits (0xffffffff) by the strtoul instead of from the lower 32-bits
(0x80010000).  The mkimage should be able to tolerate the longer
sign-extended 64-bit version of the 32-bit arguments with the use of
strtoull.  Use of the strtoll in place of the strtol in mkimage.c
resolves the issue of self hosted kernel builds for the Creator CI20
board (+) and (++).

(*) http://techpubs.sgi.com/library/dynaweb_docs/0620/SGI_Developer/books/DevDriver_PG/sgi_html/ch01.html
(+) https://github.com/MIPS/CI20_linux/issues/23
(++) https://github.com/MIPS/CI20_linux/issues/22



Signed-off-by: William Cohen <wcohen@redhat.com>