bootflow: bootmeth_efi: Set bootp_arch as hex

bootmeth_efi sets up bootp_arch which is read later in bootp.c
Currently bootp_arch is being set as integer string and being
read in bootp.c as hex, this sends incorrect arch value to dhcp server
which in return sends wrong file for network boot.

For ARM64 UEFI Arch value is 0xb (11), here we set environment as 11
and later is read as 0x11 and 17 is sent to dhcp server.

Setting it as hex string fixes the problem.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Shantur Rathore <i@shantur.com>