ARM: omap3/board: add spl specific board_init_f

gd is already setup when calling board_init_f and
preloader_console_init depends on it. Therefore move
the SPL specific part to its own board_init_f as it
prevents the need for a temporarily gd.
board_r: only assign gd when requested

When CONFIG_SYS_GENERIC_GLOBAL_DATA is not set the arch handles
the assignment of gd. At least in case of ARM/Aarch64 this means
board_init_r is alteady called with the new gd. Therefore only
assign gd if CONFIG_SYS_GENERIC_GLOBAL_DATA is defined.
ARM: do not set gd in generic board again

Since crt0.S / crt0_64.S already allocate gd and assing it there
is no need to do it again board_f / board_r as it only wastes
stack space.
ARM: SPL: do not set gd again

Because gd is already set in arch/arm/lib/crt0[_64].S,
we do not need to set it in arch/arm/lib/spl.c
cc-option: make it work with clang

By default clang will return echo a warning if an option is unknown.
Therefore turn warnings into errors when polling for options.
ARM: make gd a function a function for clang
inline: use the gcc inline version instead of the c99 one.

---
This fixes errors like:

make[1]: Entering directory `/home/jeroen/software/u-boot/arch/arm/cpu/armv7/omap-common'
arm-linux-gnueabi-ld.bfd  -r -o libomap-common.o  reset.o timer.o utils.o
timer.o: In function `get_tbclk':
/home/jeroen/software/u-boot/include/asm/io.h:81: multiple definition of `__raw_writesb'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:81: first defined here
timer.o: In function `__udelay':
/home/jeroen/software/u-boot/include/asm/io.h:88: multiple definition of `__raw_writesw'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:88: first defined here
timer.o: In function `get_ticks':
/home/jeroen/software/u-boot/include/asm/io.h:95: multiple definition of `__raw_writesl'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:95: first defined here
timer.o: In function `__raw_readsb':
/home/jeroen/software/u-boot/include/asm/io.h:102: multiple definition of `__raw_readsb'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:102: first defined here
timer.o: In function `__raw_readsw':
/home/jeroen/software/u-boot/include/asm/io.h:109: multiple definition of `__raw_readsw'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:109: first defined here
timer.o: In function `__raw_readsl':
/home/jeroen/software/u-boot/include/asm/io.h:116: multiple definition of `__raw_readsl'
reset.o:/home/jeroen/software/u-boot/include/asm/io.h:116: first defined here
make[1]: *** [libomap-common.o] Error 1
make[1]: Leaving directory `/home/jeroen/software/u-boot/arch/arm/cpu/armv7/omap-common'
make: *** [arch/arm/cpu/armv7/omap-common/libomap-common.o] Error 2
eabi_compat: add __aeabi_memcpy __aeabi_memset

fixes this:

common/libcommon.o: In function `env_callback_init':
/home/jeroen/software/u-boot/common/env_callback.c:47: undefined reference to `__aeabi_memset'
common/libcommon.o: In function `parse_stream_outer':
/home/jeroen/software/u-boot/common/hush.c:3154: undefined reference to `__aeabi_memset'
disk/libdisk.o: In function `get_device_and_partition':
/home/jeroen/software/u-boot/disk/part.c:596: undefined reference to `__aeabi_memcpy'
/home/jeroen/software/u-boot/disk/part.c:605: undefined reference to `__aeabi_memcpy'
fs/fat/libfat.o: In function `fat_set_blk_dev':
/home/jeroen/software/u-boot/fs/fat/fat.c:60: undefined reference to `__aeabi_memcpy'
fs/ubifs/libubifs.o: In function `ubifs_tnc_locate':
/home/jeroen/software/u-boot/fs/ubifs/tnc.c:1457: undefined reference to `__aeabi_memcpy'
fs/ubifs/libubifs.o: In function `tnc_insert':
/home/jeroen/software/u-boot/fs/ubifs/tnc.c:2008: undefined reference to `__aeabi_memcpy'
fs/ubifs/libubifs.o:/home/jeroen/software/u-boot/fs/ubifs/tnc.c:2333: more undefined references to `__aeabi_memcpy' follow
arm-linux-gnueabi-ld.bfd: BFD (GNU Binutils for Ubuntu) 2.23.2 assertion fail ../../bfd/elf32-arm.c:7683
arm-linux-gnueabi-ld.bfd: BFD (GNU Binutils for Ubuntu) 2.23.2 assertion fail ../../bfd/elf32-arm.c:7683
arm-linux-gnueabi-ld.bfd: error: required section '.rel.plt' not found in the linker script
arm-linux-gnueabi-ld.bfd: final link failed: Invalid operation
README.clang: build command with clang