iscsi_ibft: make ISCSI_IBFT dependson ACPI instead of ISCSI_IBFT_FIND

iscsi_ibft can use ACPI to find the iBFT entry during bootup,
currently, ISCSI_IBFT depends on ISCSI_IBFT_FIND which is
a X86 legacy way to find the iBFT by searching through the
low memory. This patch changes the dependency so that other
arch like ARM64 can use ISCSI_IBFT as long as the arch supports
ACPI.

ibft_init() needs to use the global variable ibft_addr declared
in iscsi_ibft_find.c. A #ifndef CONFIG_ISCSI_IBFT_FIND is needed
to declare the variable if CONFIG_ISCSI_IBFT_FIND is not selected.
Moving ibft_addr into the iscsi_ibft.c does not work because if
ISCSI_IBFT is selected as a module, the arch/x86/kernel/setup.c won't
be able to find the variable at compile time.

Signed-off-by: Thomas Tai <thomas.tai@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>