include bare bone spm routine

[agsb]

This include a minimal bare bones spm routine, for non arduino and assembler gcc programs.
Uses r0:r1, r30:r31, as default, and r24:r25 as command and sreg holder.
Only does spm, wait and return. Only. Could be selected instead do_spm.

[WestfW]

Why?
Effectively, it can only be used from ASM, due to register use.
It's not significantly shorter than the existing code, except for the missing re-enable of RWWSRE, which is pretty important.

[agsb]

Exact, as just an alternative of do_spm for use in ASM, and left to user do the tricks.

[agsb]

Thanks ! Your modification for _boot_rww_enable_short() solves my request in a much wise form.

// Reenable read access to flash
__boot_rww_enable_short();
7fd2:	87 bf       	out	0x37, r24	; 55
7fd4:	e8 95       	spm
7fd6:	08 95       	ret

[mcuee]

In this case, this PR can be closed.

https://github.com/Optiboot/optiboot/blob/master/optiboot/bootloaders/optiboot/boot_opt.h#L72-L82

#define __boot_rww_enable_short()                \
(__extension__({                                 \
    __asm__ __volatile__                         \
    (                                            \
        "__wr_spmcsr %0, %1\n\t"                 \
        "spm\n\t"                                \
        :                                        \
        : "i" (_SFR_MEM_ADDR(__SPM_REG)),         \
          "r" ((uint8_t)__BOOT_RWW_ENABLE)       \
    );                                           \
}))